Bad graphics (mentioned several times) has always been one of my favorites. Just because the game isn't in 18475z doesn't mean the game won't be fun.
Another favorite has always been "It looks too kiddy." I know several people who refused to play Borderlands or No More Heroes because they have cartoony graphics. Two incredibly fun, incredibly violent games that they wouldn't play because the graphics weren't "macho" enough for them. Seriously, the average "hardcore" game that people tend to think of as being for "real men" have a view of maturity that a 12 year old would laugh at. Drowning the scenery in dust and getting the VA's to imitate Christian Bale's Batman makes the game more childish than something like NMH that just has fun with itself, IMHO.