Michael Snow:
Isn't reversibility already built into all admin privileges? (Image deletion being an exception for technical reasons.)
Image deletion is quite serious. History merging isn't reversible either at the present.
Quorums are problematic because they require time to form, and part of the reason we grant privileges is to help processes go more quickly.
Yes. It all depends on the implementation. For example, automatic notifications can be used to quickly get users who are online involved. The process where I would most like to have additional safeguards is blocking. It might be preferable to have a temporary "review phase" where editing privileges are suspended, but the block is not yet finalized. The finalization could require a quorum, no objections from other sysops, or a similar process.
The reason is simple. A user who feels they are unfairly blocked once may never come back. Angela says that this, too, can be an example of an "irreversible" action. That is quite a drastic possible outcome, and we should strive to reduce that possibility while not giving one inch to spammers and vandals.
It might be a very good idea, however, if some quorum was required to repeat any admin action that got reversed. I don't know if it would be possible to implement this by technical means, but I'd settle for it as a social expectation.
Interesting idea. I see you have the right mind for working on solutions to these kinds of questions. Please join the http://meta.wikimedia.org/wiki/Wikimedia_Research_Network and help us to find and specify ways to improve the existing methods.
Breaking up the various admin privileges into their component "atoms" will not necessarily flatten the power structure. It can just as easily encourage people to collect as many of the "atoms" as possible,
Generally people should get the whole package. Atomic operations, especially when granting privileges, should be the exception. But they should be possible.
I strongly agree with your "unintended consequences" bit. Evaluation, testing, discussion, trial are essential for changes like this.
Erik