We've all seen the plethora of requests out there to have notifications added for this, that, and the other thing. Most of the people who are against such notifications are against them because they'd be annoying, and I agree with them. I'd prefer only seeing that number up in the corner for something important that actually requires my attention. This request is an attempt to create a middle ground, where users can receive such notifications that "something is up" on the not-as-vital-but-still-kinda-important stuff without the users who don't really care being irritated.
How do we indicate a user has "updates"?
The first key aspect of this request is being able to differentiate between a normal notification and what I'm henceforth calling an "update." This would work similar to how the achievements icon works, which shows a number for reputation changes and just lights up green for general badge updates, etc.
When you only have updates waiting for you, the inbox icon's color will merely change to the same red color, rather than showing a number. This will let users know that there's something they might want to look at, but it's not a full-out notification. So those users who don't care about them can safely ignore the icon in this specific case. (Perhaps it could use a less obnoxious red. I don't know, it just looks funny to me.)
How do we display them in the drop-down menu?
I'm also stealing some design decisions from the achievements menu for this one too. Just saying "you have new decline messages" or something of the sort isn't really useful unless Stack Exchange implements some global page where you can view all of these things in one spot. Personally, I don't think that's such a great idea. So instead, we can display the counts of each update under the respective site icon for wherever those updates are waiting. Each count will be color-coded depending on what type of update it is, link to the corresponding page on that site where they can view that information, and of course have a tooltip to explain what the count represents for those who are unfamiliar. Here's a basic example:
In order to not be overwhelming or irritating to users who might not care as much about them, it would be a good idea to always limit this to one line, no matter how much stuff the user lets accumulate in there. In order to achieve this, we could simply list out sites that fit onto the one line until we're out of room, and then just display the Stack Exchange logo with the number of other sites that also have pending updates yet to be seen, like so:
And here's an alternate version using grey icons instead of colored backgrounds, for those who think all the random colors would make their eyes bleed (I completely agree):
As far as sorting, there are two logical options:
Sort the sites by total number of updates for each site.
Sort sites how they are sorted in the Stack Exchange menu (Your Communities first, in your selected order, then ordered by reputation).
Personally, I think the second option would make more sense as it will always bump the updates for the sites in which the user is more interested to the front of the queue, and then they can potentially dismiss everything else they don't care about once those are out of the way.
How do these updates get dismissed?
First and foremost, simply loading the inbox drop-down will clear the red coloring off of it once it's closed. Whether you act on any of the updates is irrelevant. That shouldn't clear the updates themselves though. It's very unlikely that a user is going to get through all of those updates in one page view, which would actually require opening them all separately in new tabs. So clearing them all out after one drop-down load would be pretty counter-productive. Removing the red color indicates that there's no new updates since the last time you looked, but you should still be able to access any updates you haven't actually visited yet.
I propose clearing out the count for each update counter once the user has visited the actual page that corresponds to the count. This would work similar to how the tab counts on the profile work. Your reputation change since the last time you viewed your reputation history keeps building up until you view it again, then immediately resets to zero and goes away.
I also propose having an "x" icon (as seen in the full example above), or some sort of dismiss button, at the end of the line so users could quickly dismiss all notifications across all sites, without having to click through to them one at a time. It should probably have a confirmation dialog to prevent accidental clicking and to explain that it affects all sites, but would be very handy for users who just want to clear out all the numbers.
So what all should show up in here?
Some of the better ones I've seen that I think would make great additions to this are:
Flags which were declined.
This could link to your flag summary page. The page itself would need its long-overdue revamp in order to be sortable by declined flags, as well as when those flags got declined. It's certainly possible for a user to cast a page-full of flags and then have one from before all of those end up being declined. We shouldn't start directing users there with updates if they still have to dig through a pile of flags to find a needle in a haystack.
Suggested edits that were rejected.
This could link to your activity tab on your profile, narrowed to the suggestions option. We took a huge step in the right direction now that the list actually shows you which edits were rejected.
Posts you downvoted that have been edited.
This could link to your votes tab on your profile, narrowed to the downvote option. As I've pointed out on another post, it should be relatively easy to fix up this list and let users find "edited" posts. Each vote already has a status of whether its been locked or unlocked, and since a vote can only be unlocked by an edit to the post, you can naturally assume that a post has been edited if the vote is unlocked. A smple indication of the vote's lock status somewhere on the line would suffice - no need to look up and determine if the post was edited or not.
Questions you closed that have been edited.
This could link to your votes tab on your profile, narrowed to the closure option. Marking these, though, would be a lot trickier than marking unlocked downvotes as explained above, and I haven't really thought of any decent way to go about getting that information without creating new variables or a lot of overhead. We also have to consider when a question has already been reopened, or the close vote is not from the current closure (it got reopened then closed again) as marking it would not be relevant to that user any longer.
Something happens on a question you've favorited.
This could link to your favorites tab on your profile. I don't really think this one needs any further information.
Your posts which were recently deleted.
This could link to your recently deleted posts page (accessible from the bottom of your questions or answers list). I believe the lists are actually separated currently, but I don't see any reason not to combine them into one list and just display whether it was a question or an answer, and it would make this a lot easier to track as you wouldn't have two different pages.
Did you actually read all that?
I'm sorry, I don't really have a summary or TLDR.