20th Century mark up languages are absolutely retarded for designing web pages and breed bad layout design.
Yeah, I know I'm going to get down-voted because people lurv HTML, but having had to whip it into shape lately this was totally worth it.
Horizontal scrolling is a significant enough problem that Jakob Neilsen has seen fit to name it one of the top ten web design mistakes and include a prohibition in his home page usability guidelines. I can tell you from experience supporting corporate apps that the best way to hide something from users is to make it big, red and flashing, but make them scroll right to see it. Circles and arrows and a paragraph explaining that they have to scroll right to see the Egress are of no help at all; they have been conditioned to vertical scrolling only.
I think that is the real reason why people hate horizontal scrolling.
Reading habits of human beings are linear, not two dimensional. Trust me.