If they would make the tree-style tabs very aggressively swap to disk, or even discard state entirely and seamlessly become a bookmark that would reload when clicked (perhaps this behavior could work only for white-listed domains, like wikipedia.org), that would be perfect.
The main usability problem with bookmarks over tabs AFAIC is that the bookmark UI does not encourage pruning bookmarks, so most become stale over time.
Another idea I've been kicking around for a while is "tab decay". Have tabs start to decay and close themselves after not being viewed for a configurable amount of time. 1 month would probably do it for me, it would save me a lot of cleanup time.
