* If a class you pick feels limited in regards to weapon loadout, you can get an extra weapon type later on. Guns are good even on the power-focused classes.
* You may want to maximise either your Paragon or your Renegade score so you won't be unable to make the difficult checks that you don't know about yet. It's not as necessary as it was in ME1, but you'll still miss out on some things if you don't.
* The passive-buff skills are surprisingly good, especially Shepard's.
* When you start the IFF mission consider yourself locked into the approach towards the endgame.
* At one point you will have to choose someone to escort a group of people out of the area. For the best ending, you pretty much have to choose Mordin.