Deep water and shallow water as well. The Beaverton seminar was small, but went well. A really wide mix of skills and, as always, there are a number of things I consider basic that we didn't touch.
A lot of my purpose in teaching the way I do is to, as Joe Lewis says, "Lead them to the deep water." Until you complicate it, the physical stuff is easy. Striking two people while doing a back flip is probably hard and somewhat complex...but knocking someone on his ass isn't complex. Not unless your internal "what if monkey" gets involved. There are few things more natural and intuitive to an organism than delivering kinetic energy.
I consider that shallow water stuff, and people do need some training in it. But spending years in the wading pool doesn't prepare you for the deep end...and, to stretch the metaphor too far, many people work really hard to convince themselves that their little wading pool is the ocean. Power generation is easy. Power generation with compromised structure and injured (which, unless you are the bad guy, is the normal starting place in an assault) is a different skill.
Fighting well when you have psyched yourself up for a match is different than fighting well surprised and scared...and that is different than fighting after you have been crushed and humiliated and all you have left is a life that may or may not be worth living. I spend a lot of time on context and a little time on emotion. Partially because they are important but even more because so few people really look at them and they affect everything else.
One of the rank beginners asked a question. There is simply no way to move or stand where you are safe. Multiple targets are always exposed and every motion exposes more. If the threat has the power or a weapon, the human body is all target. He wanted to know how to defend his vulnerabilities.
I paused for a second because the question was so backward... and then we played with it. It's not about your vulnerabilities (much) it's about the threat's capabilities. He is composed of bone and meat. Any way that he stands, each motion creates and eliminates specific vectors. His weight is on his right foot? He can't kick right without shifting. Leaning away? His lead hand is weak but his rear, especially if his spine is twisted, is loaded.
Once you read the opponent, you know how he can hurt you. His options are limited. You don't need to defend everything, only what he can hurt. It's one of the reasons I prefer infighting is that people are easier and faster to read by touch than by sight, but the principle really doesn't change.
Things change by history. This isn't deep-water stuff for me. I don't think I learned it my first day of martial arts, but certainly in the first year and probably the first month. It's a basic. Getting the question from a beginner was okay... but I also got a 'thanks' e-mail from someone else at the seminar who considered this the big take-away. Not sure how I feel about that.