Here's my 2nd set of ideas.
Back the grain idea above, I once saw a program which showed end grain in a different color.
My work is very often symmetrical. I can make halves of a model in SU and combine them, but it's not as straightforward as it could be. Perhaps something which would create the same joinery on both ends of a board.
An automated dovetail tool would be cool. It should work on both pieces of wood at the same time.
Perhaps a selection of curves often used in wood working which can be scaled to fit. Bezier tools work, but I think there could be a more straight forward approach.
Easy access to some standard dimensions. I often go to http://www.woodbin.com/ref/furniture/index.htm for that. Need to make sure it will work for different standards around the world. On that same note, perhaps some scaled figures which could be posed to see how they fit into furniture. These could be components that could be added to a model.
How about a quick way of indicating where screws or nails go without actually modeling screws or nails.
Now that I'm thinking, I'm sure I'll come up with more.