ME1 was a train wreck of a game. Everything about it from the stupidly designed combat, badly handled and implemented skill system, lousy enemy and squad AI, awful inventory, bland UCWs, a difficultly curve that flattened out so fast that the second half of the game could be played with your eyes shut... the only thing that saved it was the writing. Oh and the Mako was a pointless badly implemented transparent time waster. ME1 is the only game I really loved when I played it that I just cannot stomach playing again because it is such a bad game to play.
2 and 3 were leaps ahead in terms of gameplay but faltered on the writing end.
ME1 was slated as a slow burner and it worked well, each mission and sub mission was built steadily on the one before in a sensible manner, consider, for example the tunnels when you are first ambushed by the hordes of geth after having established the basics of combat on the first planet. The boss encounters were also much better from the standpoint of layering them with unique complexity, such as the mission on Noveria against the Asari hive queen spawning thing, or even the fight against Benezia, or really just about every other fight in ME1 save Saren who you could overpower to death with just about any spec save Sentinel or a poorly constructed biotic.
Or consider the extremely well balanced intricacies of each of the squad members in ME1 having a corresponding specialization to balance out other specializations, or your own specialization, as well as the diversity of having each specialization match well against different campaign environments. If you landed on Noveria and thought you could just use a Sentinel tank approach with Garrus and Wrex backing you up well you would be wrong because the heavy biotic emphasis in that mission would force you to re-prioritize. On the other hand, if you used a biotic and other spell heavy approaches you might be ok there, but in trouble on Feros.
ME2 and 3 were taken directly from just about every other FPS out there, along with weaker balancing and tuning, and as I stated before, vast oversimplifications of fairly engaging systems.
Consider, for example in ME2, the abyssmal overtuning of the target on the first engagement with the collectors, or the end fight against the extremely cheap flying robot thingies that I don't remember what they're called. There is also the extremely over-wrought fight against the Kroga berserkers on Jedore.
I do not recall enough of the rest of 2 except to say that it was, as I said, a straightforwardly competent cover shooter, none of the missions or designs on bosses or systems were any more unique than that. I do remember the re-design of the main ship being fairly irritating compared to the elegance of the original Normandy.
ME3 in particular had weak combat design on for example the boss fight such as against the giant reaper with your laser gun which played like pong if your tiny bar was slathered in molasses.
In ME2 and 3 it doesn't really matter what your spec is or what squad mates you have, you just press the amazing button.
The biggest issues in each game were alternately the level design or also parts of systems design, there isn't one thing or another to single out.