Here at Gamereactor we have already completed Mass Effect multiple times and can't wait for the upcoming sequel.
The following is only to be considered rumors, which stem from Gamespot, whom again has removed the info from their own site.
Apparently Gamespot has revealed how it will be possible to play with up five players simultaneously in co-op. The experience level has been raised to 120, it is now possible to import your original team from the first game, and the weapon modification feature has been greatly improved.
Mass Effect 2 will be exclusive to the Xbox 360 for the first six months, and there after converted to the PC and Playstation 3.
The pictures below are from Mass Effect.