I have a 2011 Yz 450f that I love. Its a rocket! I raced it from August on after my Rm 250 tranny went. I stsrted actually winning a few motos because of the bike. I did however have an issue with the valve cover gasket leaking. It was a problem with the 2010 and they apparently fixed it bit there have been some cases of it happening on the 2011's.
I thought mine was leaking but it turned out to be excess filter oil dripping out of the airbox. I wonder how many people had this happen and thought it was the valve cover gasket leaking.