I'm not going to oversell it, but the "HD" version of the original MP looks pretty damned good.
Also I just saw a REALLY simple, enumerable example of the IQ increase on Guardian, H3. On the 360 version, I was looking at some moss hanging from a vine - there were four fronds of moss dangling vertically. On the Xbone version, there were nine. Same object, same viewpoint, just more detail actually appearing in the screenspace.
I don't know if I'll play another fps again other than MCC.
Halo 2 and 3 are gonna be major and amazing timesinks.