5.2 > 5.1 > 5.0 > 5.3 > 5.4
Isle of Thunder stands out because it was a great designed zone with a variation of dailies that had the option for PVE and PVP. It had Rare Elites, the Troves and such, and Scenarios to tell the story, which I think was better than the 5.1 version, if by a little. 5.1 was great but still.
No love for 5.3/5.4 crap. Grind grind grind. 5.3 wasn't as bad because you were limited in terms of how much you could do, and it was linked with Valor which is what MoP feels to me, a Valor grind.
5.4 makes the 200 Valor harder to get, has a crap new currency and chucks loot out to every Timeless Noob who wants to pop into Throne of Thunder or Siege of Orgrimmar LFR and cause others issues. In 5.4 any idiot can get full 496 Gear without even killing a Mob. The only thing I like somewhat is the 200 Valor Weekly Quest, even if I complete it every three weeks. The Elites are just annoying, most are dead before you can get close, especially Houlon, and I don't know what they were thinking with Evermaw. Unless your a Shaman or DK its annoying, you get killed by Fatigue and the bugger cannot be slowed. Also its the first time I've been forced to use my Water Strider, which I don't like even slightly. And that Shahao Rep can go to hell, I'll aim for it in 6.4. Has no sense of progression to Grindful Isle.
And what others have said, more Dungeons. A Troll Five Man or SoO Five Man would have been great. To me 3.3 and 4.3 Dungeons have been my favourite groups in the game.
Anyways, end of rant.