A couple of the more general gotchas come to mind:
- ASP and ASP.NET cannot share session state which makes the partial migration of applications using that service very difficult (or requiring a kludge of some kind)
- No more render blocks - in classic ASP it was common to use render functions due to the top-down nature of the page processing. ASP.NET, of course, uses an event model so at a minimum your render blocks will need to be modified.
That is but just two gotchas that popped into my head when I made this post and I know there are many more. I'm looking for some “real world” gotchas or stories that can be shared with the development community at large. If you got em then please share em!