Well, yesterday at around 6pm PST, UPS delivered my copy of Mass Effect 2: Collector's Edition. Wheeeeee! Been looking forward to this for a LONG time. Anyway, got to play about an hour or so of it, so I'll give you the rundown:

Graphics/Sound/Music
I was very, very impressed by the significant level of improvement in the quality of this area, at least in comparison to the first Mass Effect. The whole thing feels smoother and cleaner. Animations are mostly seemless, and I've yet to encounter any glitches or graphical bugs (like the popping textures from ME1). The opening scenes look much more like an actual cutscene. Character details are also improved as well, especially in the eyes (ME1 had some issues with this)

Voice acting is impeccable as usual. Jennifer Hale and Mark Meer once again deliver a stellar performance as Commander Shepard. There are marked improvements in the delivery of their lines too (Meer was often noted by fans as being overly stoic and somewhat monotone in his acting, while Hale sometimes overdid her part). The lip-syncing is nigh perfect as well.

Sound and music is spot-on too. Exactly what we were expecting from an epic title. Some pieces will sound /very/ familiar to anyone who played the original, while some of the new pieces fit into the game beautifully. Even on a somewhat crappy TV, the overall look is quite nice, though I would recommend playing it on an HD TV for best results. I did notice some aliasing issues on the 360 version, I do not know if this is an issue on the PC version.

Gameplay
This is where the bulk of changes have taken place, and to list everything would probably be a whole new journal in itself. So, we'll start with the combat, which was touted as the largest change. Unlike the original, guns now have "ammo", though the word should be used liberally, since the game gives a perfectly sound explanation for the change. It's not bullets you're loading, it's an ejectable heat-sink. Gone are the days of ME1 with Double Frictionless Materials X on a Spectre X Pistol or Assault Rifle, where all you need to do was spray the room, and where the targetting assist could nail people with nigh perfect accuracy. Now you have to carefully aim, and the damage is now location-based, making limb and headshots the name of the game. Heat sinks are quite plentiful, and as long as you're somewhat conservative and use the appropriate weapon, you'll usually never run out of ammo. Also, the heat-sinks are universal, so picking one up can give you "ammo" for any of your weapons.

You have less powers to work with than in the original, no more armor training or weapon skills, thanks to a lower level cap (30 as opposed to 60), but your powers have noticeable improvements with each skill level you take, and cooldowns on powers have been reduced significantly. This was done to make the powers you do have more important, and to encourage people to use them smartly, not just spam and then wait for cooldown to be done. All 6 classes have received improvements as well, with each getting a unique power as a means of making them distinct from each other, but without one being game-breaking or visibly superior (like Immunity from ME1)

Squadmates have MUCH better AI, and can individually be commanded to take up different locations during combat, giving you much more control over the execution of pincer attacks and flanking maneuvers. Your teammates don't seem to have the issue with running out of ammo, but they don't spray-n-pray either. Taking cover is a lot better too. It took some getting use to, since you have to press a button to take cover now (GoW style) as opposed to the original where pressing up against an object in combat mode would result in taking cover, but the cover system feels much better, as opposed to gimmicky.

Enemies have a much better AI as well, rather than the silly charge tactics of the original, and luckily, on the harder difficulty settings there's just more of them, and they are smarter, which is a significant improvement, whereas in ME1 you spent 10 minutes pinning a Krogan in the corner with bullets until his Immunity wore off.

Conversations are as fluid as ever, with the innovative dialog wheel back in full effect (har har), but this time, Paragon and Renegade no longer require you to spend your skill points to make it easier to persuade people. Your bar itself doubles as both an indicator of your alignment AND a skill. Also, the interrupt system feels very interesting as well. Basically, depending on your alignment, an icon will appear, allowing you to "interrupt" the conversation or actions of the NPC, with a consequence dependent on your alignment. These interrupts don't always change the outcome of a conversation, but they do add a LOT of flavor.

Complaints have been leveled against the new Hacking/Bypass (basically the Decryption/Electronics from ME1) mini-game, since now they no longer require you to have a tech person with you, and are far different from the Simon-Says of ME1, requiring some thinking on the player's part, though the fact that it's no longer-skill dependent is refreshing. Also, scanning planets to find resources is considered an improvement over driving the Mako over ridiculously bland and aggravating terrain, but is still a somewhat tedious task that might become annoying to many. Still, the benefits of scanning are much more tangible, rather than just receiving a sum of credits and exp.

Presentation
Mass Effect 2 exceeds all expectations in this area. If you import a character from ME1, many of the choices you did back in the original, and the missions you took, have an impact on the sequel. Most of the changes are aesthetic, like dialogue noting your deeds. Some of the changes actually confer bonuses, such as bonus resources if you did that horribly tedious Mineral Gathering quest from ME1, or having points in Paragon/Renegade based on your alignment. These bonuses are lovely extras on an already delicious cake, but luckily, people who don't import aren't penalized. It's simply an small incentive to play the original, or to be thorough in your ME1 run. There are many complaints that the shooter elements override the RPG elements, and it is true, there ARE more shooter and combat improvements, but, when compared to the original, these changes are for the best. The characters are believable and realistic displaying emotion in face and inflection. And even if you're someone completely new to the series, ME2 gives enough background information that it could almost be a standalone game on it's own merits.

Overall
Mass Effect 2 is definitely a better, more streamlined game than the original, and Bioware really outdid themselves with the changes. They even listened to the fans, re-implementing New Game+ and making new Love Interests that should surprise fans of the original. The game is considerably shorter feeling, from what I've heard, 35 hours or so, but that doesn't necessarily make it a bad thing, as anyone who spend time doing full completion runs of ME1 will tell you how horribly tedious that could be at times. While some of the changes aren't perfect; like the hacking and bypass games basically requiring you to save before attempting, as failure means no way to re-attempt (I think, I've not failed yet), and the system of using fuel and probes to scour the galaxy feel just a touch contrived, the total experience is a noticeably improved one.

I would highly recommend that everyone who can, go out and get Mass Effect 2, especially if you already own Mass Effect 1, you won't be disappointed.