Directories can now say that they don't want to be rendered in specific view types. The usage case Chris has in mind is his techblog, where the blogdir view of categories is utterly huge because it renders hundreds of entries. Because this is intended to be a graceful gentle fix, trying to view a directory in a disallowed view generates a (permanent) redirection to the default view of the directory. To avoid redirection loops, this redirection only happens if the view has been specified explicitly as a URL parameter.
(For obvious reasons, disallowed views are also disallowed in virtual directories derived from a particular real directory.)
This is done similarly to DefaultDirViews: touch a file in the directory
.flag.noview:<viewname>. Unlike default views, this is not
currently inherited by child directories.
The 'See As' page tools links also exclude disallowed view types. Right now they do so a little bit too thoroughly, in that they exclude the default view if it's also disallowed. Moral: don't do that, even though the code saves you from a redirection loop in this case.
Right now there is no restrictions on what (directory) views can be disallowed, so you can disallow Atom feeds. This is probably not a feature and will probably not be staying, although Chris may change his mind about this or just be lazy.