Until I ran the small demo app he had written.
I was excited and impressed by this demo app and with how little code it implemented bi-directional synchronization between a forms-based UI and data in XML form. Quite a mouthful, I know, but imagine you had to write one of the editors provided by PDE: some random XML file format cooked up by someone else, which is to be presented in a nice form-based UI through which mere mortals can edit the data. At the same time, the editor should allow experts to view and change the XML text itself, such that any changes get reflected in the UI.
I had always assumed that implementing this would be a major pain in the neck - you would probably write a bunch of Java classes that would be the in-memory representation of the XML data, call the verbose w3c DOM API to get the data into your Java objects, write UI code for editing the Java objects, and sprinkle in code that keeps everything in sync. Lots of repetitive code, with many opportunities to make small mistakes that would be hard to find or debug.
Enter Angelo - he took the Eclipse data binding framework and implemented observables that work against w3c DOM nodes directly, resulting in suprisingly concise code. This is one of those things that you have to see in action to "get" it, an ideal topic for a screencast.
Last night, I decided to learn how to make screencasts by using this as a warm-up exercise. The result turned out pretty amateurish. I apologize for all the "erm"s and that you can hear me breathe, but I think it's still worth watching. Click to watch the screencast (five minutes, 7 MB).
Btw - Angelo, congratulations on becoming an e4 committer!