ClientID's making javascript an absolute pain or a mess of inline asp code.
MasterPages with forms on the child. It makes things very...strange.
Finally, most developers I've met are just...subpar. I can't really explain it, they just are good at .net stuff but they lack understanding below the surface. And I should add I don't just hate webforms (I really hate webforms actually), but even the new MVC stuff is just not implemented well in real life, probably because of the developers using web forms before using MVC stuff. I think MVC is the right way to go, but developers are so completely behind its sad. You shouldn't bill yourself as ten years experience if you have spent those ten years doing nothing but falling behind on development practices. Its not physical time that matters in this profession, its all about your knowledge-base, which needs to be worked on continuously. As I said, most .net developers, are just behind (or even scared) of newer development techniques; and when I say new I don't mean bleeding edge, I mean 4-5 year old techniques. If it wasn't 2004 - 2006, it doesn't really get another look by some of the 10+ year experience people I have worked with.