His expertise, and also our private political debates. His political views were completely contrary to mine, but Dave had a fascinating way of keeping a political debate, or any contradictory for that matter, civil and fun actually.
I used to complain to him that sometimes I would prefer if we just flamed each other. Nothing like the good old days of unchecked flame wars at usenet to help one build character, defenses, and even argumentative power. It's not like these boring days of excessively eager moderation and the internet-only false pretense that people shouldn't aggressively argue with each other. He was fine with that, but only if we argued in person