Assume for a second that TJ Holowaychuk's Component project isn't the future. And then let's say that, due to support from Twitter and Google (through Yeoman), Bower becomes the de facto tool for managing and installing client-side dependencies. Whether you're using Yeoman or Bower with another build tool, you're still left with a gap where reusable UI widgets should be.
While Yeoman improves workflow, Component also tackles the notion of sharing "widgets" that contain markup, stylesheets, and code. If you read TJ's tutorials he pushes the idea of structural markup -- stripping away unnecessary markup to leave behind a vanilla slice of templates that can easily be reskinned with CSS.
With more advanced client-side workflows provided by libraries like Backbone.js, RequireJS, and tools like Yeoman and Bower, I feel like moving away from monolithic UI projects is necessary. While I like to jump start projects with jQuery UI, Bootstrap, Closure Library, or perhaps even Dojo's Dijit and DojoX, these projects are more monolithic than the modular dream promised by the Component project.
The component idea is being formalised in Introduction to Web Components, and lists Dominic Cooney and Dimitri Glazkov from Google as the editors. Therefore, the concept is being standardised, but this particular vision of components seems very different to what TJ and other developers have envisioned.
Should a widget/component library enforce ARIA, or provide tools for making accessible components? jQuery UI went through many iterations to improve its accessibility, and Dojo has documentation on creating accessible widgets.
Data Binding APIs
It feels like we're settling on binding using data- attributes, so this might be relatively trivial in practice. Perhaps the "ultimate" component library would address this, or perhaps it's unnecessary.
What Do You Want?
Let's say you settle on Yeoman as your workflow tool for client-side development. What do you think reusable client-side widgets should look like? What would be the perfect fit alongside Yeoman, Bower, Grunt, and a data binding library?