"It's rated E".
If I had to explain one more time in middle school that the ESRB rating was not a review score, I was going to hurt somebody. Though, as has been thoroughly discussed before, reviews are not the gospel.
Popularity, or lack thereof. Just because you've never heard of Disgaea doesn't mean it's a bad game. Just because everyone's heard of Guitar Hero doesn't make it bad either.
Also, graphics, obviously. In addition, the art style. I have a friend who's a huge Zelda nerd, but won't even TOUCH Wind Waker because of the cel-shading. And let's not get into the people who bash Halo because it's "colorful".
The number one stupid reason, though? The cover art. I heard somebody bash Phantom Brave for this, saying it's a "girly game" not based on anything more than seeing the box. I guess if cover art is indicative of quality, then Mega Man is the worst game ever made (old joke is old).