It's all pretty personal I guess, based on where you are in your journey. The modes demo at the Herts bash was brilliant for me, really clear and practical. If I wasn't so far away I'd be beating Nick's door down for lessons, but I'm sure it would have been common knowledge to others more advanced.
So from a personal point of view, a couple of things I've often read about, but never fully understood.
Playing behind and in front of the beat. I get playing on the beat (almost!)... but pushing/dragging is something that I'd love to have demonstrated. Not to be able to do it so much at my level, but to recognise and be aware of the difference, and the effect it has. Probably a difficult demo though as it'd take a pretty capable bass player and drummer I guess.
Effects - I've always just dabbled (and just have a B3 at the minute), but to hear from someone who knows their stuff would be interesting. I hear people who use effects here and there and it's awful, then there are those who use loads and it just sounds right. What/how to use, what to (and not to) mix, How to mix/blend, chain order, etc... and what compression actually bloody does???
Having said that, any technique stuff of any type or genre, or any applied theory stuff is always interesting too.