I personally like XForms. However, I think it is a lost cause on the popular Web for three reasons:
It's not backwards compatible with existing Web content and existing Web browsers,
It uses too many levels of abstraction to be understood by most authors.
It requires the use of many namespace prefixes.
None of these problems really have anything to do with XForms features per se. It is IMHO possible to bring many of XForms features (especially the declarative constraints ideas) to authors without introducing the other problems. For instance, by just extending HTML's form features instead of making a totally new language.
--Ian Hickson, Opera