I have not been able to find Cmd-3 (or -1, or -Space,
or -<any key>) in the manual, so this is an
undocumented bug as far as I am concerned. The manual
(actually the Quick Reference; it is not in the
manual) says that the shortcut for filtering by
rating is Ctrl-1, or -2, or whatever.
The filtering is not by rating - a quick check of the search bar reveals that Cmd-<number> searches for images by the keyword preset of that same number. The Search bar even changes from "Showing All" to a badge indicating a keyword is being searched for (the same badge used when an image has a keyword applied).
While I can't find the keyboard command documented either, do you not think it kind of handy to search for images by keyword preset just as quickly as it is to use the preset? And the command make a lot of sense since as you note Control-<number> is a quick search by rating.
Aperture as done everything it should do UI wise - they have indicated a search is under way. The only thing they've not done is document it, but it is definitely not a bug or even bad UI. The UI is fine, if you know about badges and keyword presets. And that's just the thing, with any complex application a little understanding is necessary to use it effectually. I would hate to see a very useful feature dumbed down just because someone sitting down in the first hour of use may not understand it. Why make life a little easier for the first hour and make it slightly harder for the last 50,000+? It's a matter of balance.
The correct key sequence is error-prone; a single
typing error can cause all your pictures to
disappear, apparently permanently. This is not what I
would expect of a $500 "pro" product.
As noted, the search bar changes - get used to looking at it if images "disappear" after you have hit a key sequence.
Now what is a bug is that "Undo" does not undo the application of the keyword filter. That would be my first inclination for a fix when you've accidentally hit a wrong key.