Some tech to check out: - jQuery for DOM manipulation (interacting with HTML quickly and effectively) - Underscore.js for a nice toolkit of often-used utilities - node.js for running a server, which means you can have JS on both the client and the server. - Backbone.js/Angular.js/Ember.js (and others) are Javascript MV*s: that make large, single page apps like Pandora faster, more maintainable and easier to build.
Things to look forward to: ECMAcsript 6 and 7 are in the works and include tons of new features: http://en.wikipedia.org/wiki/ECMAScript
ASM.js was recently created by Mozilla and it allows near-native speed using pure JS to run things like Unreal Engine inside the browser without the need for plugins: http://ejohn.org/blog/asmjs-javascript-compile-target/
Even more standardization. IE has finally become a fairly standards compliant browser, so JS in this browser is very likely to work in any modern browser. Coding standards are also more common and are held to more often.
Personally, the worst thing about JS in the browser is that you have to deal with the DOM. The DOM is a smelly old piece of crap that acts differently in different browsers.
The trend has been to use less HTML and more CSS to style it. HTML5 is also an attempt to extend the DOM's capabilities and also make it more semantic.