I think the combination of simple API, good performance, big company backing (proven at scale), rich dev ecosystem (e.g. good dev tools, related ideas such as Flux/Redux) and, of course, React Native is going to be hard to beat. As a freelance developer, it's the front end framework I've chosen to go "all in" with for this round - but still it's great to see all the innovation going on in this space (once you get over the fear that your skills will be outdated in six months time that is!)
I wouldn't exactly consider React "lightweight," especially compared to other frameworks like Mercury [2], Mithril [3], and Riot [4], which are a fraction of the size of React but still have their own virtual DOM implementations.
[1] https://fb.me/react-0.14.0.min.js
- React 0.14 is 55kb
- jQuery 1.11 is 38kb
- Backbone + Underscore (dependency) are 15kb
Also, by "right-weight" I think this is meant in terms of API and features, not file size. Sorry, but I don't think that bringing up the file size of various other frameworks is that productive of an addition to this thread.
Also, I would crown knockout the light-weight champion (both in function, and concept), and ember the heavy-weight champion
Only thing missing is a capable router, and I think you'd be off to the races with this thing, awesome.
I personally like the background of the main dev(s?) behind Aurelia. IIRC, they worked on knockout->durandal->angular->aurelia, and I think the apps they stopped-by in before conceiving Aurelia have just the right concepts and features to make for a really compelling large-ish framework.
A little dissapointed about how hard it is to bootstrap it though, I really like to start projects from scratch (empty folder), and found it a little difficult to just pull together easily (without downloading their starter-zip or whatever).