So as not to be attacked, let me book end this by saying the obvious: This is all just my opinion.
I like Mass Effect 1 better.
To explain why, lets start with visual aesthetic (because I just watched EC and that's what I'm thinking about). Mass Effect had a GREAT visual aesthetic. From the color palette to the architectural designs. The locations you visited all looked interesting and vibrant. Sure there were texture pop-in problems, but I didn't care. The game just looked good.
Enter Mass Effect 2. Apparently, BioWare REALLY wanted us to get that this game is DARK! A toned down color palette, darker lighting, and architecture more focused on ruin and decay. Hell, a lot of sections wouldn't look out of place in Gears of War. There were exceptions, of course. Illium is a good example, but even here, you're barely able to enjoy the fresh feeling before you're fighting your way through a darkened, partially constructed building or a gray-dominated warehouse.
And don't even get me started on the loading screens. Who thought that Orange and Lime Green were a good combination to make players look at during the game's excessive loading times?
Next lets look at the Planet Exploration vs Probing (bow chicka bow wow). I don't need to say much about the Scanning. It was dull, tedious, and a bad idea.
Yes, the Mako controlled like absolute ass, but I still preferred bouncing around in it to the scanning. The reason is going down to explore the planets gave the galaxy a much more diverse feel. It really made you feel like you were going out and exploring a vast and varied galaxy. Some of the planets were beautiful (my fav is the Silica planet where you get Wrex's family armor), while others were harsh and barren. This contrast gave the game world a very living feel.
Combat wise, it's a toss up. I don't like ME2's reliance on chest high walls and I often felt that my character's skill level was pointless. On the other hand, I felt like ME1's combat was unrefined. It just wasn't always that interesting and I felt like the character was flailing half the time. Early reports say that ME3 is going to be a combination with the focus being on less of chest-high walls, more skills, and increased emphasis on using your characters strengths to support the party in battle. Skeptical, but if they pull it off, it will be the best combat in the series thus far.
Quest and plot structure goes to ME1 hands down (I'm not talking about dialogue because both games have absolutely fantastic writing). Almost every single quest in ME2 lead to a gun fight. The same can almost be said for ME1, but they were better framed. A lot of ME2's quests lead you on thinking they were simple, but as soon as you started seeing chest-high walls, you knew something was about to go wrong.
In Fact lets stop and count which quests lead you on thinking there was going to be no combat:
Freedom's Progress was supposed to be abandoned.
Recruiting Jack was supposed to be a simple pick up.
Same with Samara.
Miranda's loyalty mission was supposed to be simple.
So was Tali's.
And Jack's.
And Jacob's.
Going aboard the derelict Reaper.
And the Collector's vessel.
That's a lot of times to use the same "It's a TRAP" trope in one game.
Then there is the simple fact that ME2's plot plays out a lot like ticking off a checklist. Recruit persons 1, 2 and 3? Check. Solve their daddy issues? Check. It might be more tolerable if the game didn't focus on Combat so much. For example, I LOVED Tali's loyalty mission, but I thought it would have been better if there had only been a token Geth presence on the ship. The game's dialogue is the most interesting part of it. Don't make us wait so long to get to it.
So, is ME2 bad? No. It just stumbles a lot more than ME1 did. Most of these issues come across as not having enough development time, but that's just speculation. It just seems to me like they put less time into it. Choosing to take a cut down, quicker route.
Oh, wait morality system..... Yeah ME1 wins. Tying persuasion to ethical alignment. That doesn't hamper roleplaying at all... :|