A mind-blowing concept that doesn't seem to exist in the default form of any browser I have come across; only in a few extensions and add-ons that don't always work exactly how I would like them to.
The ability to individually mute tabs. I'm no expert in browser-progamming, but surely if Chrome can have a separate process for each tab, muting individual tabs can't be much of a stretch. Every time I open up a range of tabs, with one having a stupid auto-playing video, I have to look through every single tab to find it.
Worse still if I have a flash game open in one tab (I know, I know, 'too old for flash games') that doesn't have a mute option I can't just mute that tab if I want it to temporarily shutup while I watch a video in another tab. Forget hover-boards, where is my tab-muting? It's 2013!