For some applications, it would be nice if Tapestry would support Tapestry 4's request cycle: action requests immediately render a markup (HTML) response rather than send a redirect.
This would make it easier to support stateless applications.
It would still be possible to get redirect-after-action behavior, by returning an Link instance from an event handler method.
I strongly oppose making this the default, since I think the user experience (i.e., bookmarkable URLs, non-repeating of form submissions, etc.) is significantly better using the default T5 approach.
This would be an application-wide configuraton value.