Playing mass effect 2 again?

If you only play through once you only get to hear half the voice content... You can go through once choosing renegade and again choosing paragon etc. You also get to "start" the later games at a higher level and get an xp bonus for completing once...

I finished the game for the first time at level 26 so reimporting that game I started off the second at level 26 with a load of credits \ resources etc already in place to make the second time through a lot faster \ easier. I bumped up the difficulty rating a little for the second play through.

In the medals case theres the medal for reaching 30 as far as I can so far thats only possible by playing htrough a second time reimporting your previous save game. Theres also the medal for completing the game twice. I get the feeling the devs want people to try both Paragon and Renegade options. I hope to see some "real" changes in the game from doing so.

Ok, I really want to play through the game again, but I have a HUGE concern.

What decisions get carried into Mass Effect 3 if you play through a second time??

I want all the decisions I made now to carry through to Mass Effect 3 (I'm full Paragon after my first playthrough). If I play again and want to try a little renegade, won't I end up killing off people that would end up being left out of Mass Effect 3??