Man, I remember wondering if Javascript would ever be widespread or enabled enough for me to even make use of it. Not to mention how useless it was until the DOM got "sorted".
Everything since then has been a kind of slow, gradual surprise.
I made the same experience, especially when I discovered jQuery and other cross-browser libraries.
Before, most scripts went along the document.write(document.lastModified);-line. And if I remember correctly, not even that worked in all browsers correctly. Made me give up rather quick.
I started working on a Chrome "Installed App" for the upcoming web store and was surprised to learn about the File API and being able to access files through JavaScript. Certainly never expected that.
1. Heavy-duty graphics, including video (canvas/svg)
2. Collaborative text editing (canvas/svg/ajax)
3. Read books in a mobile browser without a connection (localstorage)
I kind of expected a little more variety. Off the top of my head, here's what I didn't expect HTML/JS to do, in addition to the above:
4. Work on almost all new phones, without converting to a special language (Remember WAP/WML)?
5. Have open & standardized libraries that anyone can use to make coding easier (proto/moo/jQ).
6. Enable anyone to write browser-plugins easily.
7. Run as a server (node.js)
8. Constantly improve in performance
9. Be written using templating systems: SaSS, HAML
10. Still be handwritten in 2010 and beyond.