I noticed this too.. i played ME1 on a friends' 360 and ME2 on my PC and only now have my own 360 so i don't have any saves to import (and definitely don't have the time to replay 1 and 2 prior to the game).
It seems that apart from the choice who to leave behind in ME1, Alenko or Williams, the game made its' own choices what happened in previous games or i didn't notice the changes through my actions. For example a buddy of mine met Jack on a side mission and i never did whereas i had a side mission where i met Jacob and he didn't.
Just tried a MP match at gold level and my god that was hard.. way too hard for my taste but the rewards are big. Lasted to wave 8 and it's incredible what get's thrown at you at this level. And Cerberus Phantoms are most likely the hardest enemy to fight.. extremely agile and very hard hitting (every character goes down after 2-3 strikes at most).