Honestly, I can say that it isn't the lack of new content that made me disinterested in WoW.
Now they've started adding these new, improved features to make the game more accessible for everyone, like the LFD tool, and expanding battlegroups for faster queues. They made a 'guild finder' as well, and now it's easier than ever to jump into the content of the game. The levelling is streamlined and you can get to max level faster than ever. There's two world pvp areas that get objectives every 3 hours (although no-one does Wintergrasp any more, so it's effectively one), and you can get gear easily and efficiently with the Justice/Valor point vendors.
For me, that's what's making the game LESS fun. See, with every change made to the game in order to make it easier/more efficient, you lose something else. A little piece of WoW's soul, if you will. Bear with me.
The Looking For Dungeon tool, on the surface, is a great idea. Queue up, wait for a time and then hey presto, insta-PUG. Little additional reward and damage boost for grouping with randoms, but... that's not something that really excites anyone - it's just 'pretty cool'. What you lose, however, was for me much more noticeable. If you've used the LFD tool, you've no doubt noticed that in a lot of cases, people rarely even talk to each other in the party. It's all very cold and clinical, a lot of the time - people tend to just talk at you if you do something wrong. Even worse, the sense of disconnection from the people you're playing with means a lot of the time, if players think they're wasting even a second of their time, they'll leave the group without a word. What's worse, though, is when you find a fantastic group. I've (rarely) seen groups that get along really well in the random PUG, communicating, joking and having a great time. See, before the LFD, cross-realm tool, that's the sort of group that in-game friendships were based on. I've met long-lasting REAL friends in WoW, that exact way. Now? Good luck finding that nice priest again, he's not from your server - you might not see him again.
Also, never having to leave the city to do a dungeon is probably the least immersive, most boring way to deliver PvE, ever. At least when you had to fly to a dungeon there was the prospect of some world PvP outside the entrance... I'll get to that, though.
Similarly, the queue times for random battlegrounds have gone down a lot because of the expansion of the battlegroups, but I remember a time where you'd form rivalries with members of the opposite faction on your server - it was harder when the battlegroups were small, but still possible. Seeing familiar friends and foes on the battlefield brought something more to the PvP than just grinding for honour.
Guilds were formed around the small amount of effort needed to put together a 5/10/15 man raid group or a regular premade PvP group. The new Guild Finder just seems like a lazy way to push the new Guild Perks/Achievements razzmatazz recently implemented.
Finally, world PvP. Coming from a time where Tarren Mill/Southshore was a full on war, where STV was a surefire place to find either gankers or level 30-40s engaged in battles, where Blackrock Mountain was full of the skeletons of people who clashed while on the way to the many instances there, I miss REAL world PvP quite badly. Not forced world PvP, where the sides are auto-balanced, and the objectives are constructed - but PvP that was born from proximity - Horde and Alliance forced together, whether that be a dungeon entrance or two quest hubs close together. Yes, RPGs work on reward-based progression, and all these shiny baubles we're thrown are nice, I'm sure, but sometimes the reward should be playing the game itself. I miss having huge wars outside Blackrock Mountain for no reason, it was chaotic, and it was damn fun!
Also, a minor gripe, but still one of the many pieces that has made WoW less engaging for me, is the way gear is obtained. I agree, having a few pieces of gear available for some accrued points is a nice system - but having an entire set obtainable through points has completely killed the sense of excitement one could get when that piece you wanted dropped.
Maybe I'm just afraid of change, or whatever, but WoW slowly lost my interest, in bits and pieces, for the reasons described, and some others.
Damn, I ranted.
TL;DR - WoW gained accessibility, lost personality and some of the magic that made it such a popular game.
