Having just started ME2, this thread is timely. =)
I played the crap out of ME1 (though when I went to import a character, I realized my level 60 had all the plot choices I DIDN'T want to import, so I had to make do with my level 50 and do some quick modding), and there are a lot of things I like about ME2, and a few things I think were a step backward... so far.
The Positives:
-No more inventory from ****ing hell. There are a relative few number of weapons that you simply upgrade. That's a HUGE improvement.
-No more driving randomly around a damned planet looking for chunks of rock to scan. No more attempting to vertically climb hills in the Mako. No more worlds that look like the same geometry plunked over and over again with different colours.
-Have I mentioned the reduction in the number of weapons?
-Improved control interface for squad combat.
-Levels don't involve so much sprinting about across all sides of a freakin' station/planet/etc.
-The improved Paragon/Renegade options in dialog with actions is slick; much better than the old charm/intimidate.
-Leveling: 30 levels with XP based on missions makes a lot more sense than 60 levels with XP based on kills, etc. Much improved for the better.
-Powers and abilities seem more diversified and oriented to specific gameplay mechanics. My Vanguard no longer plays like an adept with a shotgun and pistol but without singularity.
-No more hacking/decrypting inanity and having to put talent points into those abilities. Nothing was so annoying as landing on a planet, running around and finding a probe, and then discovering you didn't bring the teammate with the appropriate skill. Argh.
-Flying the ship around was moderately amusing, until I discovered that fuel has been introduced (see below).
-Introducing ammunition was a good idea, though I'm not totally sold on the implementation.
-The combat, the cover system, and pretty much everything* about the aggressive gameplay is a massive step in the right direction.
The Downsides:
-STILL NO ABILITY TO SKIP THE ****ING CUTSCENES LIKE THE NEW CHARACTER MOVIE. HONESTLY, THIS FUNCTIONALITY EXISTED IN 1990 - WHY DO MODERN GAMES KEEP OMITTING IT?!?!
-I kinda liked the hunt for superior armor, bioamps, omni-tools, etc. Now it seems it's all upgrades-based rather than loot, which makes me somewhat sad.
-Related to the above, a balance could have been struck between the previous inventory silliness and the current system. I like to be able to find a few different weapons (WITHOUT DLC, THANK YOU!). Part of RPGs is traditionally collecting ****; when you eliminate virtually all but the most basic **** for collection, it makes OCD packrats like myself a little disappointed.
-Fuel? Probes? Scanning? WTF is this ****? No, it doesn't add depth to the game, it just makes it tedious to obtain resources. At least in the original when you scanned a planet you couldn't land on, you just collected the resources. This new system just adds hours to the game that could be better spent doing other things.
*My caveat to the gameplay improvement... having to strip protections before I can use my biotics makes Ryan a sad Ryan. One of the absolute best features about the original ME was being able to throw enemies across the room, float them in the air, strip away their shields and health, etc. I have fond memories of using a maxed-out Lift on Geth Colossi and roasting them with my shotgun... and if the battles against the early-game mechs are any indication, that has somewhat gone away. Yeah yeah, it makes me use a more tactically-balanced squad... PFFT. I want my biotic badassery back.
-BioWare points for DLC sucks some serious donkey ass. Let me pay cash for exactly which DLC I want to buy, not buy POINTS in some bizarre combinations and then use those to buy DLC. That's just ****ty business practices... not that you don't expect it from EA, but PC gamers don't engage in this points bull****.
-The inability to JUMP in this game, and the use of 6-inch high ledges to keep you from moving to certain ledges, is just laziness in level design. It's a nitpick, but srsly BioWare, you can let people jump in your RPGs. Enough of this tomfoolery.