哪些SharePoint开发概念对ASP.Net开发人员来说最难理解？

I'm trying to put together SharePoint 2007 (and, eventually, 2010) training materials for experienced ASP.Net developers, and having done SharePoint for years, I don't really remember where the worst sticking points were at the beginning - not to mention the amount of Googlable SharePoint content is greater by an order of magnitude than two years ago.I'm trying to put together SharePoint 2007 (and