The tl;dr based on this talk[0] by acdlite is that they're looking to separate out high-priority and low priority rendering, so you don't end up (for instance) with animations that are janking because item #12,141 in your list hasn't rendered yet. They're using requestIdleCallback and requestAnimationFrame to schedule the work. Sounds like the architecture of fibers themselves is very heavily inspired by the way a call stack is implemented.
so, quick read seems to me it will be mostly backwards compatible?
Definitely. We have 30,000 React components at Facebook so we're not planning large breaking changes.
This actually makes me question the utility of the site - the old is angular 2 production ready site had the utility that angular 2 was going to break angular 1.x so you might want to think about which you were going to use.
I guess it doesn't really matter, just the site seems less useful. The technology however seems more useful to me for that.
Yes, this was designed for our own internal purposes and is not meant to be outgoing communication. The Fiber project shouldn't deter anyone from using React.