I started WoW a few months after it went online but after about a year went to playing intermittently, usually about one month on then two off. I just noticed that last year I only played two months and a good bit of that is because of the great simplification that's been going on. Summer of 2008 I had figured that would be my last but just a couple of months later Jon at work told me that with a new patch hunters could now tame devilsaurs so of course I was back. (Though as cool as devilsaurs look I found core hounds to be much better.)
A good example is Alterac Valley. When I first reached the level to enter it was fairly complex. Each side tended to break up into teams to gather the materials for flyers or reinforcements, try to summon the big gods, round up wolves, capture mines, etc. There was strategy involved even if getting 40 complete strangers to do it was tough. One result is that matches could last a long time - I was in one that went two hours and there were believable reports of some that lasted up to six. Today, though, that's nearly all gone. AV matches tend to be races to take out the towers then the general and as a result rarely last more than 20-30 minutes. I was even in one that lasted just nine.
There are probably hundreds of other changes designed to simplify and speed up the game that mostly feels like dumbing it down. Some of this was inevitable. Increasing the leveling speed almost had to be done so that enough people would be at the cap for the expansions. The hunter mechanics were so hard to understand correctly that most hunters were played very poorly - that definitely needed to be streamlined. While it does remove some of the concrete feel of a persistent world to allow players to enter a battleground from anywhere I have to admit that I like that. And while allowing the purchase of purple gear through honor points caused an outcry from hardcore players it's the only way non-raiders like me would ever get this stuff.
But I guess what really seems missing is the feeling for about the first couple of years of openness and discovery. Though I rarely join groups and have never been in an instance raid I did participate in several city raids. The impromptu weekend battles around Tarren Mill were a hoot even if there were fairly pointless. There was a lot of crafting discussions and figuring out best approaches to gear or talents. Guilds since then seem to be less friendly though maybe I'm just in the wrong ones. (Being in a large, imploding guild was certainly less than fun even if it had a train-wreck fascination.)
The expansions drained most of that while the daily quests, jousting and world pvp took much of the rest. With players spread out there's less opportunity for ad hoc pvp (or even ad hoc help) and the battlegrounds and dailies absorb attention. In a way I suppose it's better that there's a direction but it still feels like something is missing and I'm certainly not the only person to feel that way. It was fun for a while to level characters in different races or classes but after a while you're playing through the same quest for the sixth or seventh time so yawn. (Blizz has steadfastly refused introducing random factors and events, don't know whether that's from specific design issues or a more abstract sense of purpose.) Admittedly one of the great things about WoW is that it supports numerous types of playing styles and personalities but that can't really help much after a while. Once hitting the cap everything feels more restricted but then lots of hardcore players feel otherwise. Maybe it's just that grinding the same dailies and waiting hours for an instance aren't really for me. I'll admit that the Cataclysm expansion will draw me back by essentially creating a new but still familiar game but that's probably months away.