ME3 employed different weapons with different, albiet linear statistics. Which could be further customized via mods. You can certainly argue there were too many of them, you didn't want to collect them, some were over/underpowered, etc. But that doesn't make what ME2 did any better. Just less cluttered. And given that ME3 did in fact increase the number of available weapons and brought back weapon mods indicates that the backlash to ME2's chainsaw surgery was actually listened to
Yeah, I'm shocked too 
The clutter was in fact a huge problem though. Clutter represents a resource cost, like anything else.
It's not razor thin at all
Bad RPG mechanics =ther were rpg mechanics
Ignored rpg mechanics=THere weren't any to begin with.
And what are the RPG fans who want to enjoy good RPGs going to do? In both cases, they'll go play something else, hence the razor thin margin. Yeah, we can point to ME1's inventory system and say "ME2 doesn't have this". But the net effect is the same: your RPG fan base is not enjoying your gameplay.
Let me put it this way: given all the criticism directed at Bioware for things they've done badly but did implement (the Mass Effect story works pretty well as an example), I don't think saying "at least there were RPG mechanics" is a good defense any more than saying "at least there was an ending" will make people who disliked ME3's ending happy, myself included. I think that's a very bad starting point to talk about their relative merits.
Improving TPS elements may make a game a better shooter. But removing RPG elements does not make for a better RPG. IF you like shooters, well, that's fine I guess. You'll enjoy the game more. ANd if that's what Bioware wants to do, that's okay too. THey should just come clean about it and stop claiming Mass Effect is a hybrid.
While I agree Mass Effect is a hybrid, to be absolutely clear a hybrid does not necessarily imply a 50/50 split. This is why I emphasize the ME1 = Gears of War more closely than ME1 = Baldur's Gate. I could throw up arbitrary numbers of where it ranks on the spectrum, but suffice it to say, the game is more tps than rpg with respect to game mechanics.
Again, I'm not saying there's anything wrong with wanting RPG elements and having no interest in tps elements. But I don't consider ME1 (the most traditional RPG of the 3) to have been made remotely with that fan base anywhere in mind. I'd feel much more comfortable giving Mass Effect to my tps friends than any of my rpg friends, as an example.
As for ME2's marketing claims, I admit we're quite a few years removed from that time. But I do recall them gushing about the new heavy weapons of the game as well as talking about the new smg. THus implying more options, not less.
Conceptually, that might be true, but I still think this interpretation is too narrow. My first thought at seeing the heavy weapons was "I guess we're moving more towards a tps". I'm not sure the above is enough to push the "Bioware promised to fix the inventory" argument. At least, not in the way that their marketing overhyped the ME3 ending, imports, etc.
ME1 had flaws. I'm not denying that. But what Bioware did was remove the mechanics entirely rather than try to improve them. THe fact that even as sorry a game as ME3 could actually put some of them back in and even improve upon them (though sadly gutting others at the same time) shows that they didn't need to be cut out to begin with.
I definitely think ME3 is by far the best of the 3 in terms of gameplay, but this argument cuts both ways. Saying ME3 demonstrates ME2 didn't need to use a chainsaw may be accurate, but again I've admitted to that multiple times in this thread (there were a number of options open to Bioware). But ME3 also shows we didn't need ME1's bad gameplay in the first place either. But like they say, game design is iterative and hindsight is 20/20.
The reason I pointed out the razor thin margin above is that with ME1, pretty much nobody wins in the "good gameplay" department, which is the point of purchasing a game with a specific type of gameplay in mind. Again, we can imagine the much improved version of ME2, but at the end of the day it pushed the gameplay closer towards something that some fan might look at and say "I enjoyed that".