Overall, MoP is a much better expansion than Cataclysm. The gameplay seems to have been given more thought, and there is a lot of varied content to try out. Most of the classes/specs are fun to play. I like Raid Finder as a concept. The farm was a very good idea. The theme/story is very weak, but that's not enough to make me quit.
Having given it some thought, the problem is that I don't feel like I'm accomplishing anything when I log into the game. And it comes down to two core aspects of the game design: RNG and Grinds.
Obviously, all of the knee-jerk apologists will jump in with "RNG and Grinds have been part of MMOs forever, blah, blah blah".
The point is not so much that these things exist, as it is how they're tuned. There are numerical values involved. You get (x) amount of rep per daily, dungeon, scenario, raid, etc. and (x) amount of VP. You have an (x%) chance of items dropping. Various rare mobs, chests, etc. have a respawn timer set at (x). The weekly VP cap is set at 1000. The VP cost of items is fixed. The same applies to PvP.
It all amounts to this: doing the same thing over and over again, working towards some reward, or hoping that you get lucky. The grinds have been given a much more elaborate design in MoP. It helps a lot with the problem of repetition. But the bottom line is that no matter how many different dailies and activities you put in, you still have to let players have rewards, and feel like they accomplished something.
When you dangle a reward in front of somebody, there is a point at which you lose them. When the grind to get something is sufficiently long, you feel no satisfaction or accomplishment when you finally get there. You're just glad it's over, and pissed that it took so long, and you hate the game for making you do it.
When the drop rate for an item is sufficiently low, you lose interest in even trying for it.
This lack of rewards is becoming more and more obvious in MoP. They lowered the VP cap, and the rate at which you can accumulate VP. They did not lower the cost of items. The drop rate of Raid Finder is so low that it's about as much fun as playing the lottery. The difference is that buying a lottery ticket takes about 1 minute. Raid Finder can take several hours. It's fun the first time when it's new content. After that you need to be getting something out of it. If the drop rate is not increased, your time is better spent farming VP through more efficient means, and then logging out once you've capped for the week.
Simply put, I'm getting tired of playing WoW for many hours a week and getting next to nothing out of it. The progression rate is excruciatingly slow. I have many alts. I would run them all through the same content if the rewards were much more frequent, and it was easier to get gear. Instead, they sit idle while I try to stick to one character, still making very little progress.
This character has a blue 463 weapon and a blue 463 trinket 6 months into the expansion. I got it to 90 within a few days of release, and have played it consistently. I've killed the Council of Elders in ToT 4 times in the last few days, used Runes every time, for a total of 6 rolls. Nothing. The Blacksmithing items will unlock 3 or 4 weeks after the patch, then it will likely take another several weeks to grind up the ridiculous mats for the crafted weapons. And that's just one example.
I'm just about done with WoW, once again.