Show only messages without any tag? Seems pretty clear to me.

Do you really think there should be a shortcut for something like that? This example is especially worrying because it's a security feature.

How is a keyboard shortcut any more worrying than a link? It doesn't have to be a single key, it can be a combination.

Personally, this is what drives me crazy with Gmail. I can almost, but not quite, use it without a mouse. It works for most stuff, and then you get stuck.

I moved to Thunderbird (+muttator) just because of that.