There are around 70 variables carried over from ME1 to ME2, though almost half of them have a matching opposite choice (for example one variable for killed NPC XYZ, one variable for having let them live). A large amount of these choices do little more than add flavor to the story and the universe. Some, which seemed to have little impact on ME2 (letting the Rachni live) may have a major payoff in ME3. ME3 is going to pick up some 1,000 variables from the first two games (ME2 only imports those 70 from ME1, so 900+ are coming from ME2). I think the major choices are going to see some payoff, while many will again do little more than add flavor or variance (though, really, they go a long way toward adding depth and characterization).