The only major choices that are carried over from an ME1 playthrough are:
- Whether you saved the council or let them die. Without importing a save, the council is considered to have died.
- Whether you saved Ashley or Kaiden. Without importing a save, Kaiden dies if you're a male and Ashley dies if you're a female.
- Whether you killed Wrex or not. Without importing a save, Wrex is considered to have died.
- Who you romanced. Without importing a save, Shepard is not considered to have romanced any characters.
The only other thing you miss out on are certain bonuses to XP, credits, and resources that you get from transferring a save file. But those aren't really a big deal; you'll easily hit max level well before the game is over.
A lot of people hate the idea of Wrex being dead, but he only makes a short cameo in ME2 if he did survive, so it's not a big deal. I've never cared for BioWare romance writing either and besides, you can always initiate a romance with one of the ME2 characters if you want. You only miss out if you really want to do Liara after Lair of the Shadow Broker, but if you crave hot alien tail you can always just watch that cutscene on YouTube.