Just completed this on 360 yesterday! Having never played it before, I didn't really know what to expect, but it was most excellent. I didn't really enjoy the stealth sections, but the charm and character of the whole thing really appealed to me.
I have a sneaky suspicion that this re-release it to guage reaction so they can choose to push the sequel, so hopefully it is successful, I'd love to see more!
Regarding the HD release on PC...can't you just up the resolution and settings to maximum...?