Having been able to run neither OS X 10.6 nor OS X 10.7 due to hardware constraints, I went directly from Leopard to Mountain Lion, and I have to say that for me, the improvements vastly outstrip the annoyances. Of course, I spent the first couple of hours that I was booted into 10.8 undoing many of the more controversial changes—making ~/Library visible, always showing scroll bars, and yes, returning Mail to its familiar interface (Mail -> Preferences -> Viewing -> Use classic layout checkbox).
For me, the most irritating Mountain Lion "feature" is the iCloud integration. I did use Terminal to permanently change the default Save dialog location to disk rather than to iCloud (defaults write NSGlobalDomain NSDocumentSaveNewDocumentsToCloud -bool false), but as one accustomed to frequently opening a new TextEdit document as a scratch pad, a temporary repository for random snippets of text or URLs, a way station for de-styling or reformatting copied text before pasting it elsewhere, or a means for constructing Spotlight queries without precipitating a premature search for every item beginning with the search term's first letter or two, I resent the extra click required to tell TextEdit that yes, I want to create a new document.
