About using the selective sync dialog: it takes me about 5 minutes to just load the folder list in the GUI, and it's on a quad-core Macbook Pro with 120 Mbps internet.

I believe a perfect solution for power-users would be a support for '.nodropbox' files, which would disable selective sync for a given directory and it's subfolders.

An even better idea would be '.dropboxignore' files, using the same syntax as '.gitignore'.