Cant say I remember how the combat was in ME1, but I love it in ME2. I think you can compare it pretty much to Gears of War's combat. Works very well.
The main story was a bit short (~18hrs on easy, with few sidequest done), but I think ME is best played when ME3 comes out so you can play all the games one after another, ME2 is just one chapter of the book. As much as I wouldnt want to say, ME2 lacked some of the epicness of ME1, not much, but still.
The companions are pretty cool and they certainly are different between.
You even see an old friend from ME1 joining you.
(Tho, this might not be the case if you use ME1 savegame and that character dies in your ME1 game)
Cant really say anything big is wrong with it, hell, it's Mass Effect.
But if I really must pick something then I'd say the new resource thingy is the weakest in it. It goes like this:
You have 4 resources you need to research upgrades to your armor, weapons and ship. The resources can be found during the missions, but only in small quantinies, so you must use the new planet scan thingy to move your cursor around a planet and watch this mineral thingy that spikes when you find a hotspot for one of the minerals. Then you launch a probe that extracts the minerals. Moving the cursor is very sluggish until you get the mineral scanner upgrade to your ship, which ofcourse costs minerals you need first to get.
Anywho, it's a good game and Bioware certainly is the moneymaker these days in the gaming business. Just the sheer size of the series is astounding, everything you do in ME1 affects to ME2 which then affects to ME3, the replayability value is HUGE.
EDIT: Ow forgot one other thing that bugged me in ME2. Again, something I dont remember how it was in ME1, but the good/evil system doesnt seem to have any effect on how your crew looks at you. I might be wrong, but I have pretty evil character atm and nobody seems to care. On top of that the extra discussion options you get from either being evil or good seems to work everytime you use them.
But then again, this might be a clever trick from Bioware to make players either take the easy route (evil, kill the guy if you dont get what you want or insult him/her
) or work for your goals trying to be diplomatic which then affects how ME3 will play out.
/5