The biggest unsolved problem is discovering good content. I would probably watch a lot more stuff on YouTube if I knew it existed. As is I usually learn about interesting content or producers from external sources. Making YouTube a better destination could turn it into something twice as amazing as it already is.
Also a way to block accounts would be great so I don't see "John1" "JohnsGames" "JohnsIphoneVideos" "JohnsTuesdayRants" covering the Browse page.