The "end" of an expansion is always sort of open to interpretation. You could argue that it's the expansion's final content patch, which in this case was 6.2.3, which added Cataclysm timewalking dungeons and the Infinite dragon mount (that I MUST have.) But of course, people need to play through that content, and that takes a while.
There's no clear time when players begin to step away from the game, satisfied with what they've accomplished or bored with what's there, willing to wait until something new comes around. All of that is up to the player.
So it might be easiest to say that the definitive time when an expansion ends is when the next expansion begins. I'm going to go a little earlier than that and argue that it's the next expansion's pre-patch that signals the beginning of a short (in this case about a month and a half) interregnum between expansions. You're no long concerned with the content of the old one (except perhaps as testing grounds for the patch's changes) and most of your efforts in-game will be to enjoy the pre-expansion event and prepare yourself for what's coming next.
Given that 7.0 begins on Tuesday, by this metric, Warlords of Draenor is ending in two days. This expansion has been... divisive, certainly. It has seen a sharp decline in subscriber numbers, which even led Blizzard to stop announcing them (to be fair, they had been the only subscription-based MMO to do so, but on the other hand, they had largely used it for bragging rights back when the game had 10-12 million active subscribers.)
Before we get into specifics, Warlords' biggest problem, objectively, was that there was not enough to it. There was only one major content patch, which added a zone that most of has had originally assumed was going to be in-game from the start. There were fewer dungeons than any previous expansion and none added after the launch. There were only two raid tiers (though to be fair, there were still more raid encounters than Cataclysm had,) which meant the story of the expansion felt tied up far too quickly. And despite this lack of content, we aren't getting the following expansion significantly faster than we would have in a typical release cycle.
But not everything was bad. Let's look at the expansion's various aspects.
Unfortunately, story is a place I'm going to have to knock the expansion. This is, by far, by an enormous margin, the most convoluted story that any World of Warcraft expansion has ever had, which is kind of saying something. Blizzard's motivation was to have us fight the major figures of the Old Horde, but they didn't want to simply have Garrosh go around resurrecting them, because that would raise questions of whether they were undead and if this was a kind of Horde/Scourge thing. So instead we:
Go back in time.
To another planet.
In an alternate universe.
We are removed from our familiar Azeroth three ways, and it really makes the stakes of the expansion unclear. For example, Alliance players watch as the Prophet Velen sacrifices himself to purify a Naaru. But this isn't our Velen. Our Velen is perfectly fine, chilling in the Exodar back home. One really has to wonder: once the Iron Horde was sealed away from us, what more reason did we have to stay? And how many other universes are watching the Horde massacre the Draenei?
Setting these things aside, there's also the anticlimax that is the Iron Horde. We defeated all but two of the eponymous Warlords within the first content patch, and Grom Hellscream, the headliner and Warchief of the Iron Horde is never even fightable.
In fact, what we see instead is that Grom's failure to defeat us causes the other Orcs to embrace Gul'dan's demon blood platform and he takes over, more or less turning the Iron Horde into the same old thing. Grom never has to answer for what he did, and in fact at the end of the expansion, he seems to take leadership over the Orcs again without any consequences - sure, he realizes we're the good guys and won't be attacking us again, but it's not like the Iron Horde was just purely honorable soldiers fighting a war by mistake - they were still a genocidal imperialist organization that used horrifying magic and sacrificed people to keep the Dark Portal open.
I will say that it was great to see the Draenei play a big role in the game for once - arguably being the most relevant Alliance race in the expansion - but the fact that it was a different group of Draenei who had not experienced any of the things our Draenei had kind of undercut this.
While the overall story was a convoluted mess, the good news is that the questing was very well done. The pacing was great, making sure you didn't feel stuck in one area or another. They also managed to break some of the linear quest-structure that we've been seeing since Cataclysm by often giving you different questlines to pick from, all of which would eventually come around to some climactic event at the end of the zone.
Leveling up was a lot of fun and offered some variety. One great thing was that the Alliance and Horde started in different zones (after the brief Tanaan Jungle intro,) with entirely different quest lines.
They did a remarkable job of making Draenor pretty gorgeous, and while they clearly took some liberties translating Outland back to its non-destroyed self, it was still fun to look and see how the various parts of the world corresponded to their other selves. Shadowmoon Valley in particular was an amazing place to see - possibly World of Warcraft's most beautiful zone, which makes the tragedy of Outland's fel-blasted black-and-green landscape all the more potent.
My only major complaint is: where the hell was Farahlon? Of all the zones in Draenor, this was the one I most wanted to see. It's the only Outland zone (Netherstorm, in case you were wondering) whose original name we already knew. Bah!
Players have been clamoring for Player Housing for many years. Blizzard announced Garrisons as WoW's equivalent of player housing. It was not.
Really, Lunarfall and Frostwall were just customizable cities. But they provided too much for the player - particularly the mine and the herb garden. The availability of things like herbs and ore then allowed them to complicate professions by making Tailors suddenly care about herbs or Leatherworkers suddenly care about metal. And with materials so easily available, everything became gated behind daily cooldowns.
Essentially, garrisons created a bunch of solutions that Blizzard suddenly needed to create problems for.
On top of that, there was almost no cosmetic customization for the garrisons. Alliance ones all had a clear white-stone Stormwind look while Horde ones all had red-and-spikes of the Orc look. The very thing that players had wanted - something that they could customize to feel like their own space - was instead a small, isolated plot where they'd never see other players.
On top of that, follower missions gave better gear than LFR. How is that better than the old Valor Point system?
If Blizzard ever does player housing again, they need to think of it the way that they do Toys, Pets, Mounts, or now Appearances - make it something where you can collect things for your house, including house styles, and don't force it to play a major gameplay role.
Dungeons and Raids:
The short answer is that they were great, there just weren't enough of them.
Blizzard actually did some really great work with the dungeons. Grimrail Depot remains one of the most exciting dungeons they've ever done, and others, like Shadowmoon Burial Ground and Auchindoun, had a fantastic sense of atmosphere.
But not only did the removal of Valor (later reinstated, but only for item upgrades, so kind of missing the point) make this content irrelevant very shortly, there were also just so few dungeons. There has been a downward trajectory in dungeon content for a long time (thankfully reversing in Legion) and having only eight for two years is pretty sad.
The raids were also good, though aesthetics could have been a bit better. Blackrock Foundry, for example, looked very similar to the second half of Siege of Orgrimmar, and Hellfire Citadel was kind of a fel-touched version of the same.
The fights were actually quite good, with lots of cool new ideas like fighting two guys on a set of conveyor belts or dealing with a rapidly-growing forest of mushrooms. Personally, my favorite fight was actually the very first - fighting Kargath Bladefist within a gladiatorial arena and getting flung up into the stands to fight the audience. We were missing a raid tier, but the quality of the fights we did get was fine.
Ok, not a PvPer. As a PvEer I really liked Ashran, but I'm given to understand that the people who actually like to fight other players felt it was too sprawled out and focused on other objectives. With no new battlegrounds, Ashran had a lot riding on it, and I don't know how well it did. Standing off to the side, I like that it's a PvP zone that isn't just a battleground on a timer like Wintergrasp and Tol Barad, but I can't really comment further.
Surprisingly, this is not all negative! But it's mostly negative. Like I said in the thing about garrisons, the easy availability of ore and herbs (but oddly not leather) led Blizzard to add some weird ingredients to professions that had always gotten along without such things. Being unable to craft Hexweave Cloth on my own until hitting level 96 really felt crappy.
And far worse was the fact that every single profession was limited to producing a certain amount of stuff a day. It got to a point where there simply was no reason to collect more materials, because you'd never be able to make more stuff.
The one good thing is that they managed to make the pieces you made early on the expansion retain relevance. My Paladin is still wearing the goggles he made way back in late 2014, and it's not because he's had bad luck with gear. Being able to keep upgrading those items means that your profession remains relevant over the course of an expansion, which is pretty great. Granted, requiring a specific garrison building to get the universal upgrade material (Savage Blood) is not great.
What are bringing from Draenor? Well, we did see a pretty serious "ability cull" between Mists and Warlords, though we're seeing a far more thorough one in Legion. We also got the stat squish, and to Blizzard's credit, I think it's very easy to forget that that even happened (I remember that my DK was doing 300k dps pretty consistently at the end of Mists.)
We also got new character models, which I really think were almost universally successful. I'm still not totally sold on the new Male Night Elf, whose faces seem a little rounder than they used to be. Also, my Draenei's tentacle-goatee is thicker and straighter than it once was. But generally, it's great, and I hope they one day find the time to update the Goblin and Worgen models with that new face-rigging tech.
Personally, my hope for Warlords' biggest longterm impact is that Blizzard never again tries this "short expansion cycle" concept. Not only did it clearly fail (we still wound up with about a solid year between the last raid and the next expansion) but it also felt like a kind of money-grabbing tactic to get the burst of fifty bucks from us more frequently. It's much better to let an expansion age and mature, to build up to that finale and make it feel like the time we've spent in the new area had relevance.
Thankfully, it sounds like they're taking these ideas to heart in Legion. Don't get me wrong - I'm sure there will be plenty to complain about in two years when Legion is wrapping up, but I hope that the problems are new ones and not anything like the ones we've had in Warlords.