Unless I specify otherwise, media should play in a window only when it's active, unless I specify otherwise.
Media should not play automatically by default. Again, unless I've activated it.
A bigger issue is that browsers are absolute crap for managing content generally. My typical session is: start with a few launching points (project pages, wiki/blog, newsreader, possibly social page), queue up material to look at.
At this point, my preference would be that the material I've queued up doesn't even fucking load. Too many pages and too much memory simply pigs stuff out.
Most browsers currently have a "pin as app" option for tabs -- if you want your GitHub, CampFire, Jira, Gmail, or similar pages to run as apps, then set them to run as apps.
Unfortunately, the browser has, for some time, been stuck in a worst-of-all-possible-worlds limbo: it's neither a good reading environment -- for that you'd want something like Readability, Pocket, Instapaper, or an eBook management tool such as Moon+Reader, Kindle, or (bad as it is) Calibre -- nor a decent applications environment: it's bloated, crash-prone, slow, full of security holes, and underfeatured relative to native applications.
However in both cases the browser's ability to load and display or run arbitrary content makes it convenient.
What do I want? Vastly better content management tools. Bookmarks and tab management on steroids. As Jay-Z said: I've got 99 tabs open and your autoplay isn't the one I'm looking at. If something's in background, let it fucking die already, saving state somewhere and restoring it should I happen to navigate back to it. Keeping a small number of tabs (2? 6? 12?) active should be more than enough. Least used gets offed when I switch to number n+1.
And figure out what the hell you want to do with the applications space. I do use some Web apps, but still largely find them annoying. I've all but given up on G+ and Gmail for numerous reasons, but the fact that my Chromium session becomes completely unusable once I've got either open really doesn't help matters much.
Frankly, this is like providing cleaner, brighter living quarters for slaves when what needs to be done is emancipation. I appreciate the thought, but it's far too little, far too late.