I'm sure it's been said tons of times already, but anyway -
I've been raiding since Classic AQ40 days. Recently, there's 3 things that've led me more towards quitting than any other
1) Dead servers + Paid character transfer. If there's anything that'll stop you from doing raids its being on a server with only 10 people online in major cities at a time. You'll never recruit and you'll never be able to PUG that last spot. And if you can't do this you're not making progress. If you're not making progress due to this, why pay a sub?
This isn't just invidual players that'll leave over this - it's more like people in their hundreds deserting a sinking ship. Bliz needs to fix this as first priority - I know several who've left WoW over this and won't be coming back as their mains are on a dead server.
2) Difficulty increase / badly balanced bosses. I can't be the only one to have noticed that even early bosses are now harder than heroic mode stuff used to be. 3 Drakes at Sarth was easier than Tortos normal for crying out loud. TBC raiding was the sweet spot, as was Uld normal, and ICC. The ONLY properly tuned bosses in current content are the first boss and the world boss! Now, i'm all for bosses gradually getting harder, but there's no fun Kara / ZA / Naxx style raid that you can have fun with guildies and gear up in anymore. This was one of the major failings of Cata too.
3) LFR / daily / VP grind. This is the other end of the difficulty scale. Boring and tedious, LFR simply takes too long, and feels mandatory (for the first few weeks anyway). Same with the daily quest grinds. Same with Valor. All of these give too high rewards in comparison with normal mode raids (especially normal ToES compared with TOT LFR!). Valor in particular should NOT give normal-mode quality loot.