YesConsiderably said:
Apparently there will be over 1,000 variables imported with your ME1 and ME2 character into Mass Effect 3.
If you do all of the UNC missions with the exception of the collection ones (Solarian tags, matriach writings etc) you'll be fine.
Off the top of my head, the non-critical missions that have an effect on Mass Effect 2 are helping Tali on her pilgrimage, killing the Asari consulate's sister, dealing the gang boss you meet on the Citadel and Conrad Verner.
Help Tali and you'll have an additional dialogue option when you first meet her.
If you help the Asari consulate it will be mentioned just before you start Thane's recruitment mission.
If you persuade the gang boss to give up her life of crime, you'll find her on Omega working as a social worker. If not, she works for Aria.
And you'll run into Conrad on Illium.
Everything that this guy said!
The only other thing I can say is explore everywhere! Especially in ME2 (In Illium there's a vender that sells star charts that unlocks more planetary systems there's a quest related to the vender when you run into the Asari you rescue on Feros - can't miss her she's the only green Asari in the game). And download the DLCs for ME2 I'm hoping for some ramifications concerning these in ME3
Also how did you play concerning Wrex, the Rachni and your love interests. It's a good guess whoever you shacked up with in ME will not be happy if you cheated on them! There's also a quest in ME involving a dude who was worked on by Cerberus - if you're a sole survivor you know him from your old squad that got wiped out - if he survives you get a nasy E-mail from him about how you sold out and if he ever finds you he'll kill you!