I generally choose an MMO, because my current game has become stale or boring.

I look at the class and race options, looking for a lot of variety and different combinations.
Then look at one or a few that I would like to try out.

The game has to have a decent story or lore to it.
And the world needs to be a fair size, so we don't run out of new things early.

It the game appears to be good, it is the friends that keep us in it.
There's no chance I'd still be playing warcraft after 4 years, if it was an offline single player game.
Even if I could play my assorted teams in it, and do heroics etc.

But if the GM's show they don't care about the player base (read EQ towards the end of my tenure there), the only content or the focus of the content is for a different target group of players, and most of your friends have left the game because of this... we start to look at other games.