I quit because everyone else I knew did quit as well. There is no specific reason listed, except the "I was already playing this game" (meaning it was a wow clone down to the names of talents and skills in some cases).
I even tried to look around for guilds, but eventually gave up and left and even stopped playing MMOs for a little while.
I liked Rift in the early days, lots of people, huge rift events (stupid tagging and "contribution system aside), lively world and all that jazz, then gradually the shine wore off, starting from the soul system.
I played rogue and it was enjoyable, but after a while nobody wanted bards in group content, archers sucked badly, nightblades weren't much better, so I had to jump through hoops with assassin/riftstalker/something else builds and mantaining so many self buffs via finishers it almost sounded stupid to say it out loud.
Then came the marksman patch, where I could outdamage 80% of the playerbase with 3 macros and mediocre gear, ecc.
Now, don't get me wrong, I can't even remember the exact timeline of nerfs and buffs, but for a game that got me through the flexible soul system, allowing me to do whatever I wanted (on paper)... let's just say I was quite disillusioned.
I levelled also a mage and a cleric to 50, enjoying them both in multiple specs, but as I said, playing a MMO just with a LFG tool is a tad sad.
Now, if Rift is f2p, I may even check it out again, dungeons were well done, I'd run them again a couple times.