I guess I am an old timer....this one is just like the one I shot with in basic training except for the selector switch...
I've actually fired one of the 500 or so automatic AR-15s outside of military hands. Pretty cool and the missing forward assist was definitely strange.
I prefer the newer carbines, myself. I carried the M16A1 and M16A2, but the M-4s weren't issued until I left active duty. My personal weapon is a Bushmaster A2 lower with the Colt DC6721 "Tactical Carbine" upper (A3, heavy barrel, 1/7 twist).