I think it's not so much the fact that it's long, but if you're not taking really good care of it, it's more obvious if there's length to it.

There are few things less attractive than a guy with greasy, stringy, dandruff-filled long hair, and putting it in a ponytail or hiding it with a hat just makes it more obvious.

On the other hand, if it's clean and shiny, it can be very attractive.