For me, the ability to play "and not have to DM" is a major draw. By not having to DM, I don't have to do all of the tedious die-rolling, balancing combats, or coming up with fresh dialogue or story points. I plug in, I pick a character, and then I'm off wandering around an area killing monsters and looting. While it's definitely not the same experience as playing with a group of real-life players, it can get close enough for me that I'm willing to put in the time to play.
Also, the ability to jump on and start playing without juggling 3-4 player's schedules is fantastic.
I was a rabid fan of WoW for roughly two years. During that time, I've played every race and experimented with every class. I even had some of my old gasming buddies from elementary school join in and play together for a time. However, they had more time than I to dedicate to the game, and they soon out-levelled me. Combine this with two of my old friends butting heads over sharing money in our guild, causing one of them to quit the guild and play on his own.
Ultimately, my highest level character got to 70, and then the xpac was released to up the level cap from 70 to 80 with all-new content. Excited, I played the xpac for a bout two weeks, and then I quit.
I found that the game was simply TOO LARGE for me to expect to see everything. When the guild eventually fell apart, the other players had stopped playing as well. I became disenfranchised, and as I went LFG (Looking For Groups), all people kept saying was "no one will want you on a group with the gear you have". The only way to get better gear was to participate in raids and area play, which I seldom enjoyed, because I didn't have sufficient gear to compete on any decent level. Also, I lacked the patience for farming for gold (most often by farming materials for crafting, an arduous quest in and of itself when you compete with ~1k players who may be looking for same), which meant I could never get my crafting to epic levels... which made the skills nigh useless.
In short, I couldn't keep up with the hardcore players, and I got lost in the wash.
Now I play Runes of Magic... which is for all intents and purposes mechanically identical to the WoW game engine. And it's free. All the time. And I don't feel pressured to excel or die.