On my current project, I would really like to move away from YUI and towards Backbone.js. I am a believer in using a hybrid approach of the best components instead of using a single monolithic library.
I want to move forward using a hybrid model. We would choose the best components and keep our options open to better items as they develop. I want to use Backbone.js for the app structure (models, views, router). This lets us use jQuery for DOM manipulation and progressive enhancements (not app structure). Our CSS will continue using Bootstrap components.
This is a lighter-weight approach, both in terms of library size and future development.