After trying it out:
ACP->Old revision pruning frequency, Excess revision pruning frequency
default is 0 seconds, the explanation could maybe be more descriptive, especially at 0 seconds. Seconds between running the tasks seems rather short as well, would hours or days be more fitting? Maybe the user shouldn't have to set this at all?
One thing that is mentioned is that the oldest revision is removed when things are pruned to make room for newer revisions. I think that a good option might be to always keep the very first revision of the post, and then delete old ones starting with the second oldest, if it's fairly easy. My thought behind this is that the posts in between are probably more likely to be corrections to spelling and such, and keeping the very first might be useful when looking back to see what happened to a post.
"Can view and manage own post revisions"
Are users able to delete post revisions and such with this permission setting? On my own board, I would like to allow users to view their own revisions and revert them, and possibly even see the revisions of other user's posts, but I definitely would not want to let them delete revisions. One of the reasons for post revisions is to make sure users can't get away with saying things, then hiding them by editing the post before a moderator or administrator sees it.
I would do the post revisions permissions like such:
Can view own post revisions (can view the post revisions and revert the current post back to any one of them)
Can view other's post revisions (can view the post revisions)
Can view post revisions (can view the post revisions and set any to "protected", maybe even just use the Can view post details setting for this)
Can edit posts && Can view post revisions (can view the post revisions and revert the current post back to any one of them)
Does setting a revision as protected count against it's total number of revisions saved? It could cause some problems if it does (I did not look).
I think having the feature to delete revisions might even be unnecessary. If the administrators are worried about space, more strict limits on the number of revisions saved would do a far greater job then manually going in and pruning a few revisions, which would take forever and not really do much good without significant time. Having a log of edits is one of the reasons post revisions are so helpful, so the ability to remove things should be given out carefully, and without being able to directly delete, the only option would be to flood messages until the revision limit is reached, which would be very suspicious (but then at least there would be that trace).
I would also add an administrative permission for editing revision settings, since a lower level bad administrator could fairly easily switch the revision history off and do a lot of damage after the fact.
Looks really good so far, I'm looking forward to having this feature available.