Interesting challenge

Given that lately I've been doing a lot of ETL, which involves working with a lot of low-level data structures and data transformation, I don't get much chance to think about some interesting analysis problems, particularly raster based ones. But today, I got presented with one.

Using a DEM, create a 300' buffer around any slope greater than 40%, where the change in elevation from one edge of the slope to another is at least 10', but buffer only on the upslope side.

How would you do it?  I'll post my own answer soon.