It would be nice to allow the user to select several games for tagging or hiding.

This has already been requested by many. I hope they implement this soon. Tagging large libraries is PITA currently.

What I could also imagine is "reverse" tagging: select a tag and assign games to (or from) it. This way you could easily see which games still miss the "foobar" tag.

Yep, this is the only issue preventing me from using this as my daily driver...