I agree with you lot on ME1.
Ok sure there were "arcadey" mechanics (scanning, unlocking things and so on), but that can be accepted given it was an xbox 360 exclusive for 6 months on release. And yes some of the game design for things like uncharted worlds could be refined (those 89* mountain slopes...
) but I can accept the slightly clunky mechanics for being the first in the series.
ME2 had an appalling main story led by a pantomime villain (Harbinger), stupid things like "Mission Complete" screens (my god those were awful), loading screens instead of elevators, almost entirely a corridor shooter, the introduction of bulky armour ruining that sleek retro futuristic theme of ME1, closer "over the shoulder camera" to aid in shooting (forcing our vision down the crosshairs), the split between "non-combat zones" and "combat zones" adding to the corridor shooter feel as well as not being able to holster your weapon. It totally devolved into style over substance.
I find it funny when ME2 is praised as being the best because people tend to forget the huge negative outcry and criticism from the playerbase on release at what Bioware/EA had done with the game given the expectations from ME1. I think it took several months to settle down. The whole Bladerunner vibe was lost and replaced by a Halo style.
In my opinion ME2 is the worst game of the three and the weak link because of the jarring change in style and lack of substance. It's only saving graces were that the combat mechanics were actually very good and fun to play, and the creation of some of the best characters in the series (Mordin, Legion).
The entire main story of ME2 was a waste of time and left too many unfinished plot points for ME3 to be able to wrap up decently in one game. They could have easily merged and reduced some of the characters and focused more on progressing and adding depth to the reaper plot rather than running errands and solving daddy issues the entire game, most of which didn't add much to a broader understanding of the game universe and cultures anyway.
And so ME3 ended up feeling rushed for things like the Krogan situation, the Crucible (wtf DEM device...), Rachni, and so on, nevermind that being a war story painted in one colour (thematically) it was pretty tedious. Despite having bought the DLC for ME3, I can't force myself to play through that game again because of this.
In the end I only played ME3 for the multiplayer, which is pretty damn good.