I've discussed about this many times, and it's a bit complex (I guess), right now I have so many thoughts going on in my mind but I'll give it another go. Both games have good and bad moments, it could stop there and I could just click on "Submit Reply" and call it a night, but I can't resist the urge to type too much about this (as I usually do, about almost anything).
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SPOILERS WARNING
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I believe that unfortunately the Mass Effect franchise/lore is just another victim of what I like to call the "original writer break-up syndrome" (it shares recognizable signs and symptoms of such condition with other lore and universes of fantasy and science-fiction, et cetera, whom fell victim of that as well). The first Mass Effect was written by Drew Karpyshyn, the guy who wrote Baldur's Gate II and Knights of the Old Republic too, if it helps to remind some around. But for some reason (explainable, excusable, or not, doesn't matter anymore, nor ever did matter back then apparently) he did not end up being the lead writer for the sequel, why? I don't know, but the consequences were simply inevitable, so in the end we have two games trying to be part of the same universe but they just crashed into each other and some parts of both stories managed to fuse well enough to form one entity, the rest of it is just debris scattered all around the place.
I have the feeling, of course without being sure of it, that Mac Walters (lead writer of the sequel) was just overwhelmed when he realized the complexity and depth of the universe and characters that he had to deal with and resume the narrative(s) of when he was told to do so, perhaps at first he thought "cool, continuing where ME1 stopped? sure! that would be a nice challenge for the writer that I am!", and then his mind got hit hard by the eighteen wheeler of a story that ME1 is (story part of a much larger universe that must not be neglected anywhere in the sequel). I do not believe that Mac Walters is an "amateur" or incapable writer by himself though, he surely is professional, but he's simply not Drew, and Drew had his own way (naturally) to tell the story of the main context and characters in ME1, so obviously if someone else picks up where Drew stopped there's going to be changes and differences, one can emulate a story-telling style to some extent but a professional writer don't exactly want to be someone else either, I'm sure that Mac Walters was not only given the green light to do things "his own way", but that Mac himself appreciated and actually wanted that, a writer is an artist of words and he just had the occasion to create on his own without having to tell and think like Drew would have.
Would Mass Effect 2 have been "better" than what it is right now had Drew been the lead writer? Perhaps, but we'll never know. Although logically we could easily think that if Drew would have resumed the story without the help or support of any other writer that ME2's story and the story-telling style itself would have "flowed in" much smoother than it did with ME2. I'm not sure if Drew would have killed Shepard, nor introduced a completely new enemy in the form of the Collectors, and I'm not sure either if he would have given us more information about the rather still mysterious Reapers had he been the lead writer, but I surely would like to know how it'd be like if Drew one day would come up with his own version of ME2, along the lines of "Mass Effect 2: Retold", I would actually buy it and see how different it'd end up. That's where the "problem" is, if it can be considered as such (I do) of the Mass Effect franchise, it started in someone's mind, and then it had to continue in another's, I bet that most (certainly not everyone, I'd guess, since there's always exceptions) gamers who never played ME1 and actually started with ME2, but then played the original after they finished ME2 did prefer the sequel, and the other way around might well be true as well, simply because we as gamers respectively understood and appreciated one of the two writer's story-telling style first, either we started with Drew's ME1, or Mac's ME2 (I'd be curious to know what Mac would have done with ME1 though, food for thought).
It goes beyond the "streamlining" of the game-play itself, in my opinion. I can't possibly just say that ME1 is superior because the dialog makes more sense, or because the ending gave answers to most questions we had, or because the Mako wasn't bad at all (just the terrain was). I can't say that ME2 was better simply because the combat has been "streamlined" (I'd rather use the word "revised"), or because there's more details in the graphics, or because the developers got "rid" of the inventory system and most "useless" RPG elements of the original. It's beyond that, there's those points of course, I do consider them, but what "makes" the franchise is the lore, the characters and their own stories, and the story-telling, the way the narrative unfolds, the twists and surprises in Shepard's journey, that's the story, not how you fight this or that enemy, nor even how many polygons compose Liara's ass, those things are obviously for anyone and everyone to appreciate on their own according to their preferences, and BioWare could not possibly make a sequel that would satisfy a hundred percent of the fans, the same thing applies with Mac Walters (and I repeat myself, I have nothing "against" him, I'm simply pointing at a fact that most people seem to just neglect, both games' stories were lead-written by two different writers, have that in mind when you play and it can explain a lot, it can explain both the things that do make sense and even the story holes).
Which is why, as I kept telling myself and I do tell others ever since I completed ME2, that we gamers playing both games need to remember that the persons behind their respective "architecture" are different, and so inevitably beyond the very core game-play elements themselves both games cannot be similar in any way, shape or form. Even if BioWare kept the Mako and the "exploration" intact, even if they kept the inventory system and even if they never introduced the clips system in ME2, even if the devs would have done everything in their power to make ME2 "play" like the original, it would have STILL been quite different, simply because the story of both games are. The introduction of the Collectors, the death of Shepard to seemingly "reset" the story (so that Mac could actually start somewhere by himself without the burden of keeping the continuity had it been ordered to do so), linking the Collectors to the originally-thought and depicted Protheans (by Drew) by saying that they (Collectors) were once the mighty Protheans themselves but ended up re-purposed by the Reapers during the last cycle... the decision (surely Mac came up with it, for some reason, a reason that we still don't understand so far since it wasn't explained) to make the Collectors build a humanoid-shaped Reaper of all things, seemingly ordered to do so by the Reapers themselves (or at least by Harbinger)... et cetera, all of that and more, everything that seemingly has "nothing" to do with the original (which is why Collectors never even existed in the original's story), it's just normal consequences of someone else writing a story that needs to be the sequel of something thought by another person. An analogy I like to use is as if Drew built a house up to some extent, and then decided to (or was asked to... who knows, but he probably decided to do so due to other interests and/or obligations in life) leave the place with all the architecture and construction plans, and let someone else come finish the work, but that person needs to inspect the whole thing completely anew and come up with solutions on his own to make sure that the basis is "respected" so that it holds well enough, but has no choices other than creating things to continue the work rather than just resuming it, which is what happened with ME1 and ME2, in my opinion, at least.
Discussing about the game-play itself, sure we could do that, and as I said I do like certain game-play elements of both games, and don't like some aspects of both as well, I mean I do like the Mako missions, I do prefer the "race against Saren and time" feel of the story-telling style of ME1, I do prefer Wrex over Grunt, and I do prefer the Citadel and how Cerberus really seem to be the devil incarnate in the original, I like a lot of things in ME1, but I also never really liked most on-foot side-quests, most of them were very generic (kill some foes, take items or just explore around, speak maybe to one or two NPCs, get out, rinse and repeat a couple of times), etc.
In ME2 I do prefer the combat, overall, I also like the side-quests way more than in ME1, some of which really seem to have some impact in the lore (or will have, in ME3, hopefully), I like the "main" DLCs, especially Shadow Broker... but I don't like the scanning for resources, the sudden appearance game-play wise of fuel for the Normandy (why? was that necessary?), or the forced missions (Horizon, Derelict Reaper), or the out-of-the-blue (forced, again) mini-Joker side-quest (unnecessary, in my opinion, even if well-executed, which was the case) and how it lead to that... excuse me, but I'm Shepard, and I WILL STAY in MY ship especially if a virus is going to corrupt something, say what you want Miss on-board-I-speak-a-lot computer but if something happens I'd rather have the best people around to deal with the situation at hand rather than taking EVERYONE in a SHUTTLE in the middle of no where, and to GO where? And HOW? With a *repeat it* shuttle?! No thanks (by the way Mac, one day you'll have to realize that you screwed that one up, royally, sorry but with all due respect it just doesn't make sense).
But, yeah... anyway, I guess I made my point more then needed by now, I knew this would end up in a "wall of text" but for someone who read books regularly this pose no issues, if any of you want to discuss my point of views on this whole debate feel free to do so, hopefully you read more than just the first paragraph.