Rumors say it's the same rendering and javascript engine but the UI will get more similar to Chrome and FF and there'll be extensions. Also yeah no backwards compatibility it seems, and more regular updates. Sounds good to me.
I don't think this is a good thing at all. People choose to use different browsers partly because they have different UIs, and UI changes are particularly jarring as evidenced by all the "I'll use browser Y" complaints when browser X changes something about their UI... only to be followed later by "I'll use browser X" when Y changes. There's no longer any real choice, it's just an illusion of choosing between narrowing alternatives.
Especially to see browsers' UI turning into clones of Chrome is sad, since I personally hate the "hide everything away that could possibly confuse users" and the associated "treat users like idiots" mentality that's particularly prevalent in browsers today. IE is one of the few browsers remaining that had relatively more UI, but I'm not surprised to see it disappearing... the trend seems to be to turn browsers into glorified televisions, with UIs like this one: https://news.ycombinator.com/item?id=8670503
