Any reply to this would probably be filled with spoilers so stop reading NOW if that's not your cup of tea, but obviously any character you got killed in ME1 wouldn't turn up in ME2, instead being replaced by some other (and possibly harder-to-deal-with) new character. The same, of course, is true between ME2 and ME3.Conversely, characters which would "by default" get killed in ME1 but whom you could spare by making certain choices, will turn up in ME2, if only to acknowledge your actions.
Mostly all of those choices bring additional dialog to ME2. For instance, Tali won't remind Shepard of the experiments run by Cerberus if you didn't perform the Cerberus sidequests in ME1.
The "Conrad Verner" sidequest in ME1 gives an additional sidequest in ME2 if you didn't get him killed already.
The same goes for Gianna Parasini's sidequest in ME1, which may grant an additional sidequest in ME2 depending on how you played it in ME1.
Sparing Shiala on Feros in ME1 and saving the colony grants an additional sidequest in ME2.
Depending on whether or not you romanced Liara in ME1, the Shadowbroker DLC mission dialog and aftermath changes quite a bit, and obviously whoever you romanced in ME1 (unless you got them killed on Virmire) will have a slightly different reaction to meeting Shepard in ME2 than the teammates you didn't romance.
However there are LOTS of sidequest results from ME1 that get carried over into ME2 savegames but don't yield much more than an email as acknowledgement in ME2 and thus seem insignificant. BUT some of them can have a major impact on ME3 so that's why they get carried over.
