So, maybe i'm misunderstanding, but, the perf advantage is that react waits until the next RAF and then only updates the parts of the DOM that actually changed? So, now i'm wondering: why can't the browser do that? Isn't this whole middle-man approach something to eventually be optimized out?
The browser does do this already to an extent.
The difference is that React constrains the operations a user can do (i.e. we only give them the DOM node if they explicitly ask for it and only let them manipulate it at certain times) so we eliminate the operations that cause the DOM to be slow.
If the DOM were to implement this it'd have to break backwards compatibility.
Sure, the browser could implement a virtual dom and then sync it and do a reflow once per frame.
The Angular model doesn't preserve referential transparency and therefore is inherently mutable. You mutate the existing model to track changes. What if your data source is immutable data or a new data structure every time (such as a JSON response)?
Dirty checking and Object.observe does not work on closure scope state.
These two things are very limiting to functional patterns obviously.
Additionally, when your model complexity grows, it becomes increasingly expensive to do dirty tracking. However, if you only do diffing on the visual tree, like React, then it doesn't grow as much since the amount of data you're able to show on the screen at any given point is limited by UIs. Pete's link above covers more of the pref benefits.