I spent some time in Sunnyvale speaking with roetzi and toemaz about plans to improve the revisions system in core: namely, to add in features that don't make any sense that we're missing (like the ability to 'diff' revisions), and possibly to add in the ability to provide moderated revisions, etc.
IMO, opening the opportunity to improve the core revisions system starts with de-coupling it from node module. Right now, node.module consists of some 3,000 lines of code... adding more to add in additional useful features is just asking for trouble. Further, there are many, many sites out there just using the normal node functionality and no revisions at all. Why use an extra JOIN per node_load for those people?
So please review.. It needs some work still, but especially needs testing to make sure the break was clean and all old behaviour still works.