It was Wrath that I think of as the golden age of dungeons - not only was it rewarding to run all dungeons, but it actually made raiding more accessible - you could walk into Icecrown Citadel without too much concern about making sure you had every last piece of gear out of Trial of the Crusader.
In recent expansions, we've seen dungeons really fall from prominence. In Mists of Pandaria, once you had decent enough gear to run LFR, there was essentially no reason to step into those dungeons anymore, and we didn't see any new dungeons added over the course of the expansion after having tied with Cataclysm for the fewest launch dungeons at 9 (Cataclysm redeemed itself by adding a total of five additional dungeons, though the implementation left something to be desired.)
Warlords then outdid Mists, launching with only eight dungeons and also not adding any new ones after launch. To be fair, Warlords did add a Mythic mode to the existing dungeons that provided serious gear (I have a polearm for my Retribution spec that got super-warforged to 725, which I think makes it equivalent to a Mythic Hellfire Citadel weapon.) Still, once again dungeon-running felt mostly like a side-attraction to LFR and raiding in general.
Legion looks to correct this.
It's obviously way too early to see how well it goes, but here are the new ways that Blizzard is approaching dungeons in Legion:
First off, they're launching with ten dungeons, which is a step in the right direction. I think they're playing around with level scaling to see how many of these will be "level up" dungeons and how many will be just at max level. Judging purely from item level rewards, it looks like most of them will scale to your level, with the exception of the Suramar dungeons, The Arcway and Court of Stars.
Second, they have already announced at least one post-launch dungeon, which is Karazhan. We don't know if this is going to be a simple revamp where the area is mostly the same and the mechanics are updated or if it will be an entirely new instance with its own layout and characters (though Moroes is probably going to be there, based on the trailer.) We do know, however, that it will have nine bosses and that the old raid is not going anywhere (hopefully meaning an end to destructive revamps - hey, how about giving us the old versions of Deadmines, Shadowfang Keep, Scarlet Monastery, Scholomance, and Upper Blackrock Spire... and Naxxramas, Zul Gurub, Zul Aman, and Onyxia's Lair back in addition to their updated versions?)
I'm not exactly sure what role Heroic dungeons will play in Legion, since Mythic dungeons will be available at launch (actually there might be a week delay.) But I suspect that they'll be there to help people gear up for LFR. Heroics will still be dungeon-finder-able, while Mythic will still require you to form your own group and also have a week-long lockout.
But they're also adding the Mythic Plus system. Mythic Plus dungeons will potentially scale up indefinitely, presenting greater challenges and greater rewards. This system is envisioned as a way to allow players to make dungeons, rather than raids, their main progression path, using keystones that enhance a dungeon to make it harder but also grant better rewards, including ever-higher-powered keystones.
My one concern is that making Mythic-only dungeons - such as the Arcway, Court of Stars, and when 7.1 comes out, Karazhan - means that players who cannot raid in the traditional sense for whatever reason are going to find it equally hard and frustrating to run these dungeons. In essence, Mythic Dungeons seem to be envisioned as 5-player raids - they mentioned that the Karazhan one is so long that you might decide to break it up over the course of two nights.
I'd hope that as they introduce new patches with other new dungeons, that we'll see some smaller dungeons that can be run at a normal and heroic level. Thankfully, Mythic Plus is going to mean that the dungeons will remain relevant throughout the expansion, so making something new for new players and alts as well as seasoned veterans just makes sense.
As a co-guild leader (our guild is pretty democratic) I can tell you it'll be a hell of a lot easier gathering five players together than ten, and I hope we'll be able to expand our schedule beyond a single raid night per week, potentially creating "dungeon night" as well.