I'm actually doing a lot of work with ASP right now (writing a book with a friend on .NET 3.0 ) and I find the biggest issues are twofold:
- While WebObjects is an environment where you are developing a software application where the User Interface happens to utilize HTML and a web browser, ASP.NET is all about developing a website that happens to do some application like stuff from time to time. (e.g. the session object is a big dictionary that cannot be extended, page navigation and pushing objects around from page to page is a lot more difficult, you'll find you end up doing a lot more work in the area of processing events - more like a windows desktop app, etc... )
- ADO and the way data access and controls are managed do not (easily) allow you to separate data access from business logic (ala the creation of true Entities) so you will need to either (a) build that overhead or (b) - more likely - just learn to live without it.
That being said, it is possible, just plan on missing a lot of the really convenient stuff you get out of the box with EOF and WO because of the adherence to a MVC point of view...