I too would like to see this implemented. @Oded's response to a different question lays out the SE reasoning:
We want to let you know about good things as soon as they happen -
that's what our real-time notifications are for.
Negative reputation changes are... well... negative. So we are not in
a hurry to let you know about them.
I strongly disagree, and all the comments and upvotes there and here appear to as well. I would much rather see notification of downvotes than upvotes. I get it - the drop-down is called achievements. But reputation losses do show up inside the "achievements" drop-down, so it wouldn't be unprecedented to highlight them where we can see them without having to click.
- An upvote is (should be) the status quo. It's a little pat on the back.
- A downvote means something's wrong. Maybe I wasn't clear in my post. Or maybe I misread the question. Often, this should be an action item.
Unless someone comments (which we all know isn't mandatory), I may leave an innocent questioner out in the cold, or otherwise miss an opportunity to help.
Is the purpose of the achievements display to stoke egos, or to make the site better?
If this feature request would be a major shift in the definition of "achievements", then so be it. A response of "things are the way they are because that's how they are" isn't a compelling reason not to change it.
If, and this is a huge if, the fear is that new users will be crestfallen to see notification of downvotes, then enable this only for users with the "Established User", "Vote Down", or "View Close Votes" privilege or something. But I again feel this is valuable and pertinent information for everybody, regardless of what sentiment it brings. Besides, brand new users only start with 1 rep to begin with, can't lose rep, and as such won't see this display.
This happened to me just yesterday, and I think it perfectly articulates the value to seeing lost reputation:
- I answered a fairly standard question here. Nothing to write home about. There were no other answers, I don't have experience with the framework OP was asking about, but I knew what the error meant, so explained what was happening. OP asked for some clarification, I responded to the best of my ability, and walked away.
- Presumably my explanation wasn't sufficient, so my answer was voted down. I didn't notice this for five hours.
- On finally realizing my answer didn't yet help him, I did a couple minutes of research, found the solution, and edited my answer.
- That was exactly what he needed, and the downvote was converted to an upvote.
If I'd seen a notification of the downvote, I could have helped him earlier. At the end of the day, that's what this is all about.
Questions get a mix of upvotes and downvotes all the time. But when an answer gets a downvote, it can often be improved, and will be improved sooner with this display.
The second highest voted status-declinedfeature-request of all time, shows clearly the possibility that downvotes prompt users to improve their posts.