Huge upgrades from OOTP10, and I think it's definitely worth it no matter what version you have. This coming from a huge, huge fan of OOTP10 and someone who spent countless summer hours on that game.
OOTP11 drastically improved their drafting system. Making it more realistic with signing bonuses, signing difficulty with certain players, and including their college stats which also says whether or not their competition was strong or not.
Also includes the Facegen feature which I think is gold. No more trading for, for example, Nick Swisher to the Braves and seeing him still wearing a Yankees hat. Great idea by Markus or whoever thought of that.
There's tons more, but these are just what always stick out to me. Also they improved the speed of the game (much faster), the efficiency of downloading add-ons, and improved the interface of the game overall.