I was worried about the exact things you were saying. I'm very convinced that Halo is a console FPS that simply wouldn't feel right on a PC, just as a lot of PC FPS don't feel right on consoles. You should try the XBox version.
IGN had an article a few months back asking the overall question "is Halo going to be too easy with a keyboard and mouse?" By what your experiences say, I fear they're right.
-Lasereth