Subtle Basemaps: one of the basic problems with mashups has been putting data over a map designed for route planning. Recently with Google maps API v3 and tools like tile mill people have started de-emphacizing background maps so that the data stands out better against the background map. I agree with James that the OSM bright minimal style is a interesting development in this line of work.
I'd love to see some user test results to see how people fared working with a mashup based on this base map.
Learning Curves: Also via James I read a great quote about teaching OS mapping software to students.
The rest of the post goes on to explain why Sophia thinks TileMill for windows is a game changer.