Disabling dislikes wouldn't make YouTube a nicer place. It would stifle criticism.
It's like when people upload controversial videos, only to disable ratings because it has 90% dislikes.
I don't know, I think disabling dislikes would be a bit more positive. If a video didn't have a lot of likes than it could be assumed it wasn't well received or it just wasn't good enough for people to click a button to like it.
It'd stop people from giving views to videos just to bandwagon dislike them.