WoW is, still, the best MMO by any metric of measurement. Players paying subs, update quality, update frequency, new content added, etc. Obviously the quality varies from update to update, but on the whole, WoW right now is *vastly* better than vanilla WoW was.
Questing is far more streamlined, so much so that some areas are hardly recognizable and many were completely overhauled (some more than once); class/spec viability has never been better than it is currently with MoP, and it's certainly far better than the hilarious joke that was vanilla or the tragedy that was BC; quality of life changes keep getting added to the game, with shorter hearth cd and easier access to faster mounts and mounts at earlier levels; PvP is still a joke, but let's be honest with ourselves, it always was (at least now it doesn't revolve around how much your priest can lean on his mana burn key); PvE raiding content is every bit as good as it always was, with the 14th tier of raiding a huge increase in quality over the 13th (dragon soul was shit), and PvE solo play is far more involved than it previously was in any expansion, with more interesting storylines, plot points and lore bits to gather (including a faction devoted solely to it), and even an entire brawler's arena for 1v1 challenges with tough PvE mobs.
In addition to being the best MMO on its own merits, Blizzard is quick to ape the good ideas brought forth by other games. For example, Rift launched in early 2010 and featured, among other things, rogues that could move at full speed while stealthed. This was a positive gameplay change (from WoW), and it was fairly popular. Magically, the next WoW patch that rolled around removed the movement penalty for rogues in stealth. Several games came out with AoE looting (swtor, rift, etc) and blizzard put in AoE looting.
If WoW has any major fault, aside from tiny balance issues or design decisions, it's that the gameplay hasn't evolved. This isn't to say it's gotten worse, because it obviously has not - it just hasn't done anything different. Every expansion repeats the same formula; level up, rep grind, heroics grind, raid grind. It's been the same since BC (so everyone that says the game got worse after BC, this is why; not because the quality dropped, since it didn't, but because you got tired of the formula).
TLDR: WoW is, right now, all the good things it started with, minus a considerable amount of the shit it started with, plus a lot of innovation from blizz, plus a lot of great ideas stolen from other games that came along in the meantime.
Questing is far more streamlined, so much so that some areas are hardly recognizable and many were completely overhauled (some more than once); class/spec viability has never been better than it is currently with MoP, and it's certainly far better than the hilarious joke that was vanilla or the tragedy that was BC; quality of life changes keep getting added to the game, with shorter hearth cd and easier access to faster mounts and mounts at earlier levels; PvP is still a joke, but let's be honest with ourselves, it always was (at least now it doesn't revolve around how much your priest can lean on his mana burn key); PvE raiding content is every bit as good as it always was, with the 14th tier of raiding a huge increase in quality over the 13th (dragon soul was shit), and PvE solo play is far more involved than it previously was in any expansion, with more interesting storylines, plot points and lore bits to gather (including a faction devoted solely to it), and even an entire brawler's arena for 1v1 challenges with tough PvE mobs.
In addition to being the best MMO on its own merits, Blizzard is quick to ape the good ideas brought forth by other games. For example, Rift launched in early 2010 and featured, among other things, rogues that could move at full speed while stealthed. This was a positive gameplay change (from WoW), and it was fairly popular. Magically, the next WoW patch that rolled around removed the movement penalty for rogues in stealth. Several games came out with AoE looting (swtor, rift, etc) and blizzard put in AoE looting.
If WoW has any major fault, aside from tiny balance issues or design decisions, it's that the gameplay hasn't evolved. This isn't to say it's gotten worse, because it obviously has not - it just hasn't done anything different. Every expansion repeats the same formula; level up, rep grind, heroics grind, raid grind. It's been the same since BC (so everyone that says the game got worse after BC, this is why; not because the quality dropped, since it didn't, but because you got tired of the formula).
TLDR: WoW is, right now, all the good things it started with, minus a considerable amount of the shit it started with, plus a lot of innovation from blizz, plus a lot of great ideas stolen from other games that came along in the meantime.