It's safe to say that one of BioWare's primary goals in the development of Mass Effect 2 was to fix everything complained about in the original. Mass Effect is still one of my top RPGs in recent history despite complaints about certain aspects: a hard-to-navigate inventory system, repetitive and time consuming Mako exploration, and combat issues. These were nothing major, but memorable frustrations nonetheless. Mass Effect's primary strength was in its storytelling, but that doesn't mean BioWare isn't trying to improve on that too. In short, with Mass Effect 2 BioWare wants to fix everything that was wrong with the original, and to improve upon everything that was right.
Maybe it was just me, but combat in Mass Effect sometimes felt loose and it was hard at times to get a bead on certain enemies. Mass Effect was a shooter, but it didn't quite control as well as other shooters out there. Mass Effect 2's combat instantly felt tightened up and appealed to my natural FPS skills. A new thermal clip ammo system has been added to Mass Effect 2, rather than the recharge system used in the original, so now weapons can only fire a certain amount of times before they run out. It works just fine and was added to allow better weapon balance. Gunplay has definitely been improved in Mass Effect 2.
"In short, with Mass Effect 2 BioWare wants to fix everything that was wrong with the original, and to improve upon everything that was right."
BioWare didn't forget about combat abilities other than firing mechanical weaponry. Biotic powers and abilities are easier to use than ever as they can now be mapped to hot keys/buttons for both Shepard and companions, meaning less interruption and pausing during combat (I actually found that the control scheme felt a bit better on the 360 controller rather than the PC keyboard, so PC gamers may want to whip out the gamepad if Mass Effect 2 supports it). In fact, powers and abilities are a big category improved upon in Mass Effect 2. BioWare told us that each class is meant to fulfill its own unique true role, rather than being a mix-and-match of various shared powers. The Soldier will become a bad-ass that can equip the best weapons and armor, with abilities that activate bullet-time and enable him to use freeze/fire ammo. The Adept class can now direct where it throws enemies and creates dangerous singularities in the middle of a group. These were the only two classes exposed to us, but they both played very differently so there's no reason not to trust that this is BioWare's goal with all six.
I loved the story in Mass Effect, and it only appears to have been improved upon in the sequel. Storytelling is now more organic in Mass Effect 2, but also more dynamic. Rather than going from story segment to combat segment and back again, Mass Effect 2 naturally transitions between these segments. Quick-time button events may pop up during conversations, enabling you to save a dying soldier and such, but you can miss them if you're not quick enough. Characters feel more lifelike through active camera use, different staging during cutscenes, and prettier graphics. In other words, the best part of the first game may have gotten even better. Every single story choice made in the first game can be directly loaded into Mass Effect 2 to boot.
As for the maligned inventory system, I wasn't able to experience much of it. BioWare is trying something different, with players only able to change equipment while near a weapons/armor locker. The one weapon I picked up specifically pointed out that it was the upgrade to a previously owned weapon, so BioWare is obviously trying to make things easier there. It doesn't appear that we'll be scrolling through dozens of upgrades anymore, but I didn't collect enough equipment to find out.
Elevator loading in Mass Effect never bothered me, perhaps because I installed the game onto my hard drive (through the legal method), but so many people complained about long elevator scenes that it needs to be touched upon. Load times are back, and longer than ever... just kidding. BioWare now zooms out to a wire-frame area view during loading, supposedly shortening load times significantly. They certainly seemed much quicker during the demo we were shown. Streaming content directly into the game is supposedly what made elevator loading take so long in the first game, for those of you who it bothered.
Even with the somewhat limited demo BioWare enabled us to play, Mass Effect 2 really impressed me. As soon as I sat down, I didn't want to stop playing. I wanted to keep evolving my characters, I wanted to find out more about the storyline, I wanted to try more classes, and I wanted to destroy more robots using my cool new powers. BioWare said that it is improving upon every other area of the game too: the Mako, planetary navigation and exploration, and a brand new Normandy (where you can now collect fish in a fish tank as a side game). If you liked Mass Effect, you're going to want the sequel without a doubt. If you didn't, you might want to check out Mass Effect 2 anyway, due to all of the improvements made.