Is the "Tab Center" feature a prototype for an official version of tree-style tabs, perhaps? I've seen that Servo/browser.html was experimenting with tabs-on-the-side, I'd love to see this graduate to more than just an experiment.
Potentially! The simple story is that the current tabbing model in was designed to save you from needing half a dozen browser window open. It just doesn't scale well past a dozen tabs or so, and we know that some users have dozens or even hundreds open at the same time. Tab Center is taking a fresh look at the problem with that in mind.
The tab UI works because it is part of a larger 3-level system. There are tabs, windows, and virtual desktops. With 10 at each level, you can handle 1000 tabs... except for the performance.
It's important to avoid running things on pages that aren't in focus or even in view. It's important to avoid walking data structures that scatter nodes all over the address space, causing swap access and cache misses. Watch your RSS. Keep those extra tabs idle. Make sure the "Esc" key and the stop button actually work, stopping everything (all tabs, all video, all animation, all audio, etc.) until the user explicitly asks for something to run again.