Sometimes (like today) I see that an answer of mine that had been accepted is no longer the accepted answer. Thing is, I have absolutely no idea which one and I'd really like to see which one to satisfy my curiosity. Please add that to recent history or something.
I asked for this a while on UserVoice - I'd be very much in favour of this. It's weird to see your rep drop by 15 and have no idea why. It also means there's an answer with more information in than mine, which means I may be able to learn something from it :)
Starting with the next build, this will show in your reputation history, shown as a -15 on the day it occurs (except if it's re-accepted in the same day)
I've seen this happen to me a few times where the OP unaccepted my answer and wrote his/her own very specific answer and accepted that. Perhaps this happened because my answers tend to generalize the problem, sometimes abstracting the examples, reducing the snippet, etc. This is my way of looking at the "bigger picture" and try to write an answer that is most useful to everyone, instead of OP's specific problem, but like I said this sometimes backfires with unacceptance (or no acceptance to begin with).
I've also seen question askers that reviewed all the answers received, and compiled it into one super-duper awesome answer and accepted that. I actually don't mind losing acceptances this way, because I believe we want to encourage this behavior from ANYONE. Even better if we get the original question asker to do this, I think.
I've also been tempted to do this with my own questions (i.e. answering my own question with super-duper compilation of everyone's), but I don't think I've ever unaccepted an answer because I posted my own, for fear of repercussions etc. I usually still post my answer anyway, because I think we want the content.
If my answer ever gets more votes than the answer I've previously accepted, then I may consider accepting my own. This hasn't really happened yet.
As this is apparently somewhere between difficult and unfeasible, maybe we can change people's ethos so that the emphasis is on the community to let people know why their answer is no longer worthy of being accepted.
Maybe de-selecting an accepted answer and not posting a comment to the user who provided the answer could carry a rep penalty itself if the answer has been accepted for a certain length of time. Or at least a prompt to suggest that a comment would be courteous.
Since some months (maybe a half year) we again can't see any negative change in our reputation, only the positive ones.
Its probable reason, that SE users tend to emotionally over-estimate the real worth of such small losses. Also this is the reason, why we can't see if we were downvoted any more. I've even heard from a 100k+ user who deleted his account because he got a systematical hird downvoting after he nominated himself to a mod!
As a side-effect, it makes much harder to excite the participants of a heated debate by voting them down on the meta site. It was a quite common phonemenon some years ago: you initiated something one the meta, you got a flamewar, around 20 down to your meta question and around 5 to some of meta ones.
I think, the SE wants peace. In peace, we work: we create interesting content from interesting things. In flamewars, we don't.
But it would be better if we would have a realtime feedback.