At an accessory level, tooling and ecosystem richness — which I don't really hold against Vue, because it'll definitely get there in time.

At a slightly more existential level, I miss how lightweight React components are: they're literally just functions, which makes breaking them out really frictionless. If I've got a FooWidget and I want to break out the header of the widget out into a new component, it's incredibly trivial to do — as opposed to Vue, which forces you to create a new file for it, register that component in the namespace, etc.

(I wrote about this a little more in depth here: http://jmduke.com/posts/what-vue-needs-next/)