Glorious day.
I had to play on SD last month on Battle Canyon. That map is waaaay too busy on SD, I couldn't distinguish enemies from distance scenery objects. In HD you can pretty much see other people before they can tell you're shooting them.
edit: I was in HD for most of Halo 3 and Reach, but I only recently in the last few months went to widescreen HD. That was another upgrade in visibility.
Also check your PMs once you're not busy with movin'
edit2: went from playing via VGA on a 20" 4:3 monitor at 1024p to 16:9 1080p LCD 23". I got like 3/4ths of my desk back.