I've spoken at a few conferences over several years. I feel (and the feedback seems to reflect) that I've had some good talks and some that maybe weren't so good.

IMHO, perhaps the most important thing is to know your subject inside and out, upside and down. Given that, you can speak freely and ad-lib colorful comments to fill time if needed. You are also a much better Q&A responder.

The soft skills count for a lot, too. But I think super-deep knowledge is a must-have.