Eurogamer has an article pointing to artist Matt Rhodes' blog, where he recently put up some early conceptual character designs and talks about some of the ideas that got tossed around in the early stages, and throws in some DA art for good measure. It's some pretty cool stuff. One of the things that stood out to me is he discusses an abandoned Nietzschean twist where Shepard begins to slowly adopt Reaper tech (ala Saren) and is confronted by Ashley/Kaiden. I don't know if anyone remembers, but in the early days when they announced the very first game, there was a bigger focus on organic vs. mechanical beings - in particular, they talked about how you could enhance your character with cybernetics and stuff, and the game would examine how far you could go while still calling yourself human.
In any case, the artwork is definitely worth a look-see, so go check it out. If nothing else, know that Mass Effect 2 came this close to having a legit fetus-assed Robo-Fetus as a Final Boss, instead of just a giant Terminator manbaby.