Offers much more gameplay satisfaction, the story is great (retcons excluded) and is a fantastic experience.
Powers have been vastly simplified: there are now only 30 levels, with up to 7 upgradable Powers and passive Charm/Intimidation. There are not enough points to upgrade everything even at level 30, so players are forced to prioritize. Based on how you look at it this is either a good thing or is oversimplification. I would say its a bit too much, but not a game killer.
From a combat gameplay standpoint Mass Effect 2 is comparable to Gears of War, which it tries to emulate with the same virtues and vices. There is no blindfiring and active reloads but with biotic/tech powers and power-activated ammo types it has its own style to it. This makes combat much less of a mess and with the usual cover-based challenges to it that can be overcome by Powers.
The story itself is fantastic, though in retrospect it does contain a fair amount of retroactive continuity. Various aspects of the game universe conveniently changed over the 2-year gap but thankfully none of it is jarring enough to hurt it.
Mass Effect 2's plot revolves around a proxy-war by the Reapers though the Collectors, who are abducting entire human colonies on the fringes of organized space for an unexplained reason. Bioware should be commended for another fantastic story arc to an already epic saga and fans will not be disappointed. The biggest story issue is that sadly the effects of choices from ME1 are only cosmetic by seeing old characters and by hearing some extra dialogue. These are always great to uncover but are wanting of something more substantial to the game after these moments. In this case it is now all up to ME3 to show if these will bare any effects and be worth the effort players put into role-paying.
If you are still reading then damn you are committed. All I can really say at this point is that on its own merits Mass Effect 2 is a fantastic game, one well worth anyone's time if only once.