I know, some of those features will be hidden but for example, one of my favorite things of MoI works that way, holding ctrl while using a tool like rotate makes a copy instead of just rotating the object. Another one is copying a point of a curve will make a copy of that curve with that point modified. Those are hidden features that I use a lot and if I discovered them I think more people did it.

This aren't in the "hidden" or "modifier-key-needed" group:

Selections at the browser
Make objects work like styles
Cycling shortcuts

For the hold and tap shortcut I was just wondering, nothing important.