Legion, WoW's sixth expansion, has the potential to be the best expansion yet.
I realize that saying this is kind of tempting fate, but let's look at the math:
We have a new hero class - the last time we had a new hero class we got Wrath of the Lich King, which is regarded by many (not all, of course) as WoW's best expansion. I certainly thought it was the best, but that's a mix of my really liking the Scourge as villains, the Death Knight as a class, and the fact that I had a guild that was super-active at that time, running multiple ICC raids per week.
Having a hero class alone wouldn't be enough to make the expansion good, but with the Demon Hunter we've had a boiling down of classes to their essences - again, not everyone will agree, but while talking with some guild members the other day, we were all surprised at how everyone seemed to be enjoying how their classes were playing. I had some reservations about Protection Paladins, but now that I've gotten used to it (and learned to love Blessed Hammer) I'm on board.
The expansion's content is challenging but also doesn't have difficulty spikes. It's not terribly hard to get geared up for heroic dungeons thanks to World quests and then it's not terribly difficult to go from heroic dungeons to mythic dungeons. And mythics really feel like a similar step up from heroics as heroics felt from normal modes. The hardcore might be disappointed in how easy mythic dungeons are, but we have yet to see how mythic+ winds up working out.
Leveling up is not all that time-consuming either, and thanks to the level-scaling, you can avoid certain areas if you can't stand them. There are also a huge number of relatively hidden side quest-chains that you'll be able to discover as you explore.
World Quests take advantage of the level-scaling to make daily-quest-like content that takes you all over the continent. While a lot of these are basically repeats of quests or bonus objectives you did while leveling up, the variety of rewards and, well, the variety of objectives and setting make them a step up - and for the most part they feel very "do as many as you want," and not like some forced obligation (though your class campaign will force you to do some.)
I'm of two minds about profession quests - on one hand, it's way more exciting to learn a recipe by doing some related task and it's very nice that my Demon Hunter doesn't have to go through all the old content to work up his leatherworking skills. But I got a bad first impression thanks to some engineering quests that were hard to complete - either because you could only do one literally in the middle of a boss fight (though I think they may have fixed that one) or another because it sent you into Court of Stars, which requires weeks of work to unlock.
Storywise, the stakes are high, and they've done a very good job giving everyone a sense of what we're doing here. The class campaigns in particular let you do something that feels appropriate to your particular character. Weirdly enough, my Death Knight's class campaign involved raising the dead to fight the Legion. My Demon Hunter finished his today, and oddly enough it involved invading a demonic world.
The real question is how well they can sustain the expansion. Warlords of Draenor felt pretty good while we were leveling up, and there were some strong dungeons at launch, but then we got nada for about a year.
The fact that 7.1's Return to Karazhan was announced before Legion even officially launched bodes well - getting a new dungeon (and having done a few Mythic dungeons the mythic-only tag on Karazhan is less scary to me now) as well as a new raid (oddly before the Nighthold comes out... so does that make Nighthold a 7.1 raid?) and more stuff in Suramar (which has been a great questing experience, though I'm a little daunted to start doing it on alts) is all welcome stuff.
The way Blizzard talks about the expansion, it would seem they have a Mists of Pandaria-style progression plan, which would be really great, assuming they can pace it a little better (hold things back a bit so that you don't get 14 months of nothing new while we wait for the next expansion.)
Blizzard's old goal of releasing faster expansions led to thinner and thinner expansions. Now that they have abandoned that goal, I'm really curious to see what they do when they know an expansion is going to be out for at least two years.