The biggest offender is thermal clips, ostensibly a geth invention, they are found all across the galaxy in ME2 even in places cut off from civilization like Jacob's father's colony. Cerberus was a self-interested terrorist group in ME1, but became selfless and humanitarian in ME2, before ultimately becoming human extremists in ME3. The prothean beacon visions had the collectors inserted in ME2. The Geth from ME1 are later discoved by dreadnoughts, who use kilometre long mass effect guns to bombard each other. Close in 'knife red (miraculously) to have been entirely from a splinter group, and the real Geth are largely dissimilar to the ones we interacted with. The Reapers needing to collect large quantities of humans and eat their goo was not a retcon or anything from ME1, but it was certainly a let down. Biotics morphed from relatively simple generation of blunt forces in ME1 to psychic-by-another-name in ME2 (you can even get a bonus power that grants Shepard mind control). In ME1, your shields can only stop damage caused by fast moving projectiles - which is presumably your mass effect generators slowing incoming bullets and shrapnel. Melee attacks, acid, explosions, etc. will all damage your health directly. In ME2 and 3, that is ignored and your shields provide full protection from all threats.

I am a huge fan of the Mass Effect series (I am currently 25 hours into another run of ME3), but the original poster is right. ME1 was trying to be decently hard sci-fi, assuming you bought the premise. Later entries in the series largely abandoned that, and instead focused on just using standard sci-fi cliches (I guess because few people picked up on all their hard work in ME1, so they judged t not worth it).