Half the video games in the entire industrie run on the same bloody Unreal engine for years now. Graphical quality varies greatly though, and is still improving (kinda).
You saw the same thing previous gen; Xbox stayed the same, Playstation improved a lot. Xbox is so easy (in comparison) to program for that you can make full use of it's potential with relatively little experience. Playstation is a pain but games improve a lot graphically as developers get more experience with the platform (same deal with Nintendo).
As for Reach; it's a Halo game on the 360. What did you expect?
Seriously though, it's the last Halo game coming from Bungie. Imho a nice half-priced "Halo-improved" game would have been more appropriate as a parting gift, but between the raving fans and Microsoft, there's not a soul that wouldn't pay full price for it.