It improved gameplay, characters and graphics at the cost of coherency, plot, logic, world building and immersion.
ME1 definitely had its flaws, like the long elevator rides and every base in space looking the same inside. We all know that this is primarily due to a technical limitation of Microsoft forcing developers to have the whole game on a single 6.8GB DVD. The concept of ME1 was littered with ambition and passion.
ME2 on the other hand had that 6.8GB limit removed, and yet it is more a corridor shooter than anything else. ME2 got the same treatment as DA2. Somehow people see the flaws of DA2 but not ME2.
Everyone loves to mention the Mako, but no one ever mentions planet scanning in ME2, which is at least ten times worse.
Coherencey I will grant you, but as a trade off for better characters and multiple excellent short stories, I personally vastly prefer that approach. It is very much an anthology game. As such, plot is very much a matter of taste. The overarching Collectors plot may have been flimsy, and a step down from the elaborate murder mystery of the first game, but several of the side stories are at least on a par quality wise, with far more variety while dipping into different genres.
Logic is also a very dodgy road to travel down with some of the leaps of nonsensical illogic just as present in the first game.
The series was science fantasy since day one, with biotics basically being magic, element zero a universal plot macguffin and Saren's plan, the Thorian, Rachni and Prothean plots all requiring massive suspensions of disbelief and unbelievable coincidences to work, leaving major explanations for how things worked deliberately left mysterious because it was just bollocks.
As for world building and immersion, well that's soemthing ME2 passes on the first game from a great height on.
ME1 was great at the big ideas and flashy lives of rhe people at the top of society, but ME2 showed us the darker underbelly of the universe, from Omega and it's seedy underbelly outside Council space, through dodgy dealings and assassinations, exploration of Asari legends and law enforcement, lost colonial missions, seeing the universe from the Geth's perspective, going through Korean puberty rites, Quarrian special ops missioms and court drama, going full Batman with Garrus against the Galaxies gangs and learning about rhe genophage and how Salarian's see their role on the galactic stage, ME2 took the pretty but static image of a clean, star trek like future and showed us the seedy dirty underbelly that stretched into every dark corner and good intentioned road to hell imaginable.
World building is arguably ME2's greatest strength infact, because it sacrificed it's main plot for it.
And for me, that's far more immersive. ME2 made a deep, detailed, living breathing setting, that showed you how the people below those top few elites or dramatic individuals on the edge of society actually lived.
Sure, it was a corridor shooter, but so was the first game, it was just that it's corridor shooting was shit.
And everyone always brings up how shit planet scanning was. Its was such a notoriously awful part of the game it even gets referenced in the Commander Shepherd song.