There has been some debate online about whether Mass Effect 2 represents a badly-needed overhaul or a horrible bastardization. Proponents of the former tend to accuse proponents of the latter of simply hating change, while proponents of the latter tend to accuse proponents of the former of simply hating RPGs.
I feel that I am unusually well-positioned to weigh in on this debate because I played Mass Effect 2 (hereafter referred to as "ME2") before Mass Effect (hereafter referred to as "ME1"). Thus, in my case, ME1 represents change rather than ME2.
WARNING! SPOILERS AND STRONG LANGUAGE AHEAD!
I'll be blunt: I like ME1 better. Before I begin, I will make a few concessions to ME2. Yes, all the elevators in ME1 were annoying (although ME2 randomly deciding to take a long time to load is annoying, too; see below). Yes, the Hammerhead handles much better than the Mako. Yes, ME1's hacking/lockpicking/archaeology/flag-planting minigame can be a pain in the ass, especially when it decides to be all laggy and unresponsive, which is about 70% of the time. Yes, the romances are better in ME2 because you don't have Ashley thinking she and Shepard are dating or something because he talked to her a few times without being a total asshole. ME1 IS STILL BETTER!
Better sense of progression
You level-up frequently in ME1. This is balanced by needing a lot of skill points to max out a skill. The end result is that you feel like you are making progress, more so than in ME2. Also, you can find newer, better weapons, armor, and modifications as the game progresses. While this does tend to cause the sort of inventory micromanagement that Yahtzee hates, it does provide a tangible sense of progress. Every time you find a better suit of armor or a better gun, you feel like you have accomplished something. In contrast, ME2 offers only one better gun of each type. Yes, there is a second better assault rifle, shotgun, and sniper rifle, but only Shepard can use these (unless you pay for upgrades that allow one other character each to use the better shotgun and sniper rifle).
Game universe feels big
In ME1, the Citadel is huge. There are a whole bunch of elevators (aka loading screens in disguise) because it's so huge. In ME2, it's just three small floors plus the Presidium (one room with a nice view) and a few mission-specific areas.
In ME1, almost every star system has a planet you can land on. Yes, the Mako handles like a shopping cart in a bouncy castle, but you get to drive around in a big open area and explore! It makes the game universe feel big, unlike ME2's succession of differently-colored shooting galleries. Plus, I like landscapes, and ME1 has plenty of interesting ones. Oh, and the Firewalker DLC does NOT cut it. For one thing, you can't revisit any areas you explored with the Hammerhead, and there are only a handful of such areas to begin with. Also, the Hammerhead's missles have a mind of their own and won't always fly toward the thing you're aiming at, unlike the Mako's cannon. On the other hand, the Hammerhead handles MUCH better, and, despite being a hovering vehicle, can still run over geth. That never gets old. Personally, I like Yahtzee's idea of exploring planets with jetpacks. Mass effect technology is all over the place; who's to say one couldn't make a jetpack using it? Heck, Schlock Mercenary once featured a bikini that enabled the wearer to fly. This is science fiction, damn it!
Note to EA: do not interpret this as an endorsement of jetpack-bikinis for ME3. If I see somebody in ME3 flying around in the vacuum of deep space wearing nothing more than a bikini and a breath mask, I will let Wrex eat you. Consider yourselves warned.
In ME1, visiting all the planets in the game world costs nothing but time. In ME2, it costs you credits. In addition to feeling a tad stifling, this is detrimental to the aforementioned sense of bigness.
In ME1, you have Urdnot Wrex in your party. Grunt may be a krogan, and he may have some good lines, but he just isn't the goldmine Wrex was. Stuff like this never happens with Grunt:
In ME1, you can keep firing until your weapon overheats. In ME2, you have disposable heatsinks that effectively act as ammunition. NOT AN IMPROVEMENT. Especially when you can only hold 11 or so shots for the shotgun.
Shit makes sense
In ME1, everybody wears armor. In airless or otherwise hostile environments, everybody wears a full helmet. In ME2, Miranda wears a catsuit all the damn time, and her hostile environment gear consists of a breath mask. Don't fucking tell me her genetically-enhanced eyes can withstand hard vacuum and chlorine gas. Piss off. Same damn thing with Mordin. Jack wears even less. Grunt wears a full helmet, but leaves his biceps exposed. That's excusable, since he's a krogan. Garrus and Shepard are the only ones who wear full armor with a full helmet in hostile environments. For some retarded reason, though, Garrus keeps the suit of armor that got shot up by the gunship instead of getting a new one. WTF?
CHANGE OF CONTROLS ARGCSADG$#@$#%YE^
In ME1, the R key throws a grenade. In ME2, it reloads your weapon (see above). In ME1, holding down the shift key causes Shepard to sprint. In ME2, it pauses the game. In ME1, pressing the space bar does nothing. In ME2, it does one of four things depending on context and whether it is held down or not. In ME1, pressing the E key activates an object. In ME2, it does nothing.
WHY THE FUCK DID YOU FUCKING CHANGE THE FUCKING CONTROLS LIKE THAT, EA? FUCK!
BUGS BUGS BUGS ARGCSADG$#@$#%YE^
Sometimes, ME2 decides that it will take a long time to load areas, regardless of what it needs to load. Restarting the program fixes it. Sometimes, Shepard gets stuck in midair for no good reason. The only solution is to load a saved game. ME2 DLC apparently screws everything up. I once had to play a section of the endgame three times because the first time, a floating platform didn't stop in the right place and Shepard ended up falling into the abyss, and the second time, Legion's AI glitched out, which prevented him from moving to the position he needed to move to in order for me to progress. I think telling Legion to use its sniper rifle was what triggered the glitch. Bah! The ME2 character screens have gotten quite laggy recently. I blame the DLC. During the last part of Garrus' loyalty mission, there's a bizzare hall of mirrors sort of effect in the background. WTF?
When I wrote this, Mass Effect 3 had not yet been released. I expected ME3 to continue the downward slide, and I planned to buy it purely out of a sense of obligation. I felt compelled to see this through to the end. My full reaction will be in my Mass Effect 3 review (which will be publish much more promptly than this one), but the short version is this: "I was pleasantly surprised. Then I wasn't."