My favourite feature? No contest - it's the JavaScript SDK.
So my entry is a video that helps get you up and running with making plugins for yourself.
Part 1 is now online here - and I'm rushing to get Part 2 completed before the deadline <img src="smileys/smiley11.gif" border="0" align="middle">
<UPDATE: 19 July in NZ - but still 18 July in the UK>
Part 2 is now online here. And with my eye on the flying monkey, I've given my entry a snappier YouTube title.