Of the Mass Effect series, the soundtrack is one of the most impressive things about it. The Mass Effect 1 soundtrack is the one and only I've ever bought of any game, movie, or otherwise publication of medium I've ever purchased, and it looks like that at the rate things are going, it will be the only one.
When Mass Effect 1 was made, Bioware didn't anticipate it becoming as massive of a hit as it has. At least, I don't think they did. It didn't have a budget large enough to earn it the title of "Triple-A title" and wasn't backed by a huge publisher yet, so it was more focused and niche-y, and in ways, unique.
The synth-heavy soundtrack of Mass Effect fit the atmosphere perfectly, and kept me interested, entranced, and more importantly, immersed. It seems like the soundtrack fit the game perfectly, and nothing anyone could've said or done would've made it better fitting. It also is the reason I have such a ridiculous synth fetish now.
But with the advent of Mass Effect 2, ridiculous amounts of funding and the support of monolithic superpower publisher EA, Mass Effect 2 spread itself out more to fit pre-defined roles of massive titles. Along with a few voice acting decisions, gameplay tweaks, and other miscellaneous additions, the soundtrack was swapped out completely with something that you'd expect to hear in a cinema somewhere. Mass Effect 2 had almost a movie soundtrack, and sure, there's still a few synthesizers here and there, but it completely trashed one of the most unique aspects of the original. It's epic, sure, I won't deny that. Some of the pieces are simply magnificient, such as the music at the end of the game. But that's what it's supposed to be: really, really good. Most big, heavy-budget titles, both in the film and gaming industry, don't like to take risks. They go with what sounds good, and it usually works, because why fix it if it isn't broken?
So to reign in my ridiculous rant and keep it somewhat close to the normal size for the posts on this thread, I'd change ME2's soundtrack to be more closely resembling that of the first. The synthy, spacy, electronic soundtrack was one of the reasons the game was so unique and good, and changing it out for something that's guaranteed to work still doesn't have the effect or something that works perfectly with what it's being applied to.
