Thankfully I tend to research titles quite intensively before I buy them so I usually avoid most bad games. Which rather hinders what I can select as worst of 2010 as I cant realistically pick one which I never even played.
As such, I would have to say that the worst I played in 2010 would have to be Star Trek Online.
For such an enormous license, with such a huge wealth of lore (almost half a century of lore) and background, with a scope of potential covering external starships, internal starships, away missions and the inevitable open warfare of Star Trek, it simply missed what it could have been by light years. It ended up simply feeling like Champions Online but with a Star Trek theme to me, when it could have been a quite stunning sandbox MMO. They've worked hard on it over the year and its improved, but in the long term it may well end up as too little , too late and perhaps its shuttle has already flown.