Currently the only way to exclude from dropbox sync is via the GUI as far as I understand. This is extremely cumbersome, because if you are writing code in a directory there are many reasons to want to exclude folders/files such as "node_modules" which can be quite large and hundreds of files while syncing the rest.
Adding a ".dropboxignore", similar to a ".gitignore" file that enables direct or glob-like pattern matching exclusions would go a long way to enhance the experience for users, as well as significantly reducing the burden on your servers.