I'm not sure if you asking exactly this, but here's what I'll tell you. We're building a frontend interface for our webapp now (almost finished) and I have to say it's a difficult design task, in terms of code, of course. The truth is that libraries like jQuery have really nothing to do with building interfaces. They have a couple of effects, a couple of useful shortcuts (like selecting dom elements easily), a couple of solved issues (like browser compatibility), but if you're going to write an interface - be ready to give a good thought about how to arrange the code, so that it would not look ugly.

Especially because we're talking js now, I think it's a hell of a task, as I have this impression the language was designed to duplicate as much code as you possibly can. It's not Python or Ruby and you have to understand it inside out to make it right. In Ruby, for example, it's easy to write nice looking code even if you're a beginner. I haven't tried ExtJS or some other frameworks, maybe they provide some necessary background that helps you organize things better. But jQuery and Prototype definitely don't do that. Then there's the question of testing, which is a pain in the ass in javascript, even though we have nice tools like jspec.

So I'd be really careful. Chances are you'll end up with lots of crappy code in js that would be hard to maintain, especially if your frontend is big and you don't have experience in creating interface frameworks.