: I think the biggest issue is the gamma-incorrectness of the motion blur. Halo
: 3's MB is nearly the exact same system, but it consistently looks like
: smooth blends, whereas the MB trails in Reach often look like single
: ghosts when the game is in motion.

It's because it's not supposed to be motion blur, if it was you'd see more afterimages.
It's temporal anti-aliasing.
In traditional anti-aliasing where you average together the lines where two objects meet on screen by figuring out what relation they have in screen space and looking at their brightness and color. In temporal anti-aliasing you take the current frame and the previous frame and average them together with a focus on averaging edges (and I think also touching up textures through some complicated math). When things move slowly (like just walking around) and/or the engine is able to keep up with the motion really well (like going downhill in a Warthog without much else happening) it looks fine, it's when the engine is pushed near its limits (like say a big team match on a large map) that you start to see two separate images. I think that, much like Halo 3's lighting engine that ended up limiting texture detail and resolution, if they tried it again today it'd work out a lot better.