Everyone lives and everyone shows up in ME3 to help you. Mordin has an especially crucial part to play in the final game, although he will be replaced with someone generic if he died in ME2 I suppose.
You also have to do the loyalty missions in the right order in ME2 or you risk Jack and Miranda and Tali and Legion's arguments causing them to lose loyalty if your Paragon/Renegade scores aren't high enough when they have their arguments on the Normandy. So you should save one of each pair's missions for last before the suicide mission. I always do Jack and Legion last.
Also in ME2 you can romance Tali. Best part of the games