I stopped almost a year ago, simply because I ran out of content. Did all the big story lines, discovered everything, checked out all instances and raids (except Malygos).
I already stopped being a hardcore player about 2 months before WotLK, had a burn-out and a pause of a month, then played WotLK very casually. No heavy raiding, no farming, no repeating instances ad nauseum. I really took my time to get from 70 to 80, fully enjoying all the big quest lines, fully enjoying the incredibly immersive world with a very fun guild.
2 Months after WotLK was launched I pretty much ran out of content and got kicked from my guild because 'they didn't have room for a Friends rank any longer' because they went 'hardcore' again. Fun fact: the guild split apart after I got kicked, I was a very old, well-respected and well-liked member, even though I didn't raid regularly or even play thát often anymore.
Nowadays I check mmochampion.com every now and then if something cool happens on Azeroth. I must say that sometimes I do long back to WoW, especially when they released the Argent Tournament and Ulduar (I love Titan lore). I may visit for a month or 2 when Cataclysm hits, but I'm starting to doubt that more and more. The only MMO I really look forward to is Star Wars: The Old Republic, and if that hits even slightly near Cataclysm, I drop Cataclysm like a brick.