Is it a clean site? As in the web files are completely OOTB? No custom dlls, didn't copy over dlls, etc?
Any use of non-supported code? If you used any non-supported Apis in 9 and carried them to 10, there is a good chance its not going to work at all because the dlls, namespaces classes all changed and the only thing that was left with backwards compatibility would be supported Apis.
