In many ways, Mass Effect 2 is more of the same. The writing is still far above average, the graphics are impressive (usually,) and the gameplay is fun. BioWare has (as usual) done an excellent job building a world to play in.
They've also condensed and refined many things from the original game. The first game had a clumsily implemented, cluttered inventory: the new game fixes this by eliminating the inventory entirely. You still collect and research a few new weapons, but you only carry about two to four at a time, leaving the rest at home on the Normandy--and there are far fewer weapons and armor to choose from.
Similarly, Mass Effect 1 had quite a few sequences in which you had to drive the Mako vehicle across alien landscapes. Between sluggish controls and empty terrain, Mako missions tended to be frustrating and tedious, with the majority of the "action" involving driving from point A to point B and trying not to fall into too many craters. Rather than make the Mako missions more enjoyable, Mass Effect 2 has eliminated the Mako altogether.
Character advancement has, in my opinion, been overly streamlined. Experience points are only earned by completing missions, and you only have about 5 powers per companion available to upgrade throughout the game. That leaves less room for customization than the previous game.
ME2 is not without its own boring sequences. In particular, it's important to scan every planet you find for minerals, a process which involves slowly dragging a a cursor blindly around a globe, basically waiting for your metal-detector to beep. You'll be doing that a lot, and it's not much fun.
However, all those losses are easily countered by the strengths introduced in the new game. If you have a character in ME1, you'll be able to import them into ME2, along with all the various choices you made in the first game. This will have fairly massive effects (see what I did there? Ha. Ha.) on the events in ME2...and probably ME3 as well. For the most part, the game adapts surprisingly well, although there were a few moments when a character doesn't respond as you might expect given your actions in ME1 or the events at the beginning of ME2.
Action is also more, well, actiony. They've removed dice-rolls from firefights, so if you hit someone with a bullet, you know you'll do damage: raising levels mostly increases how much damage you do, the methods by which you deal damage, and how much damage you can take. They've also replaced the old "guns-are-energy-based-and-need-time-to-cool-down-after-firing" system with a more standard ammo system, which again speeds up the action quite a bit, while also forcing you to manage your ammo wisely.
The result is that we've transitioned from Mass Effect's fairly clunky combat to a faster-paced cover system, in which you spend most of your time running from cover to cover, ducking behind things and shooting when you can, and doing your best to avoid being flanked by enemies.
Cooldown on abilities also seemed shorter, so issuing orders to your squad-mates has become more useful (and therefore necessary) than in the previous game.
Overall, ME2 is a slicker, faster, prettier version of its predecessor, yields a satisfying continuation of the established narrative, and is really hard to put down. If you're looking for more of the RPG trappings from ME1, like inventory management, stat juggling, and so on, you'll be mildly disappointed, but it's hard to be disappointed in the overall product.