I'll second the last one :)
Besides the obvious (ship/test/upgrade to 5.0) ones, I'd say documentation is *extremly* important. All the plethora of vaorious how-to's that many people were posting here and there how each particular core module works/can be used/can be abused -- would be great to have it in a single place (a Knowledge Manager?).
Remembering ACS3 days, I'd say that it was a lot easier to jump the wagon back then -- even if things were a lot less elegant. Now it is quite tough -- and there are no guarantees :)
