Make validation accross entities easier. Something like Workflow rules engine, with dependencies between rules, dynamic loading of rules etc.
INotifyDataErrorInfo would make me a LOT happier. At the moment we have to fire PropertyChanged events for properties whose validation is affected by other properties so that WPF will refresh the validation.
As we can't tell the difference between real PropertyChanged events and ones that are just there for validation we end up triggering *more* events (to refresh validation on other properties) and end up in massive PropertyChanged event storms which actually noticeably slow down our application in some cases.
I think vaidation should be like in Silverlight RIA services