"Unsync this folder" as a context menu add-on is a no-brainer.
Leaving an Alias behind, greye out, would be a perfect reminder that it's still there.
I was looking for a workaround and there are few, if you can limit yourself to linux or mac, but ultimately went with bittorrent sync ... http://peter.grman.at/ignore-node_modules-in-dropbox/
I also desperately would like this feature. It would save money to Dropbox as it would not have to continuously synchronize and keep unlimited backups of temporary files each time I do a compilation in Visual Studio for example.
I totally agree with you that it is very trivial to implement.
Of course we could do it ourselves IF there was any API to hack with, but there is none for the Windows Desktop client. And the CLI interface on Linux is very limited. Having to build an alternative desktop client (assuming it were possible without sacrificing a lot of other features) just for this feature would be ridiculous. Ideally Dropbox could release the client as open source so we could tinker with it.
I have lost all hope in this matter, considering how they shut down Votebox a while ago and have not implemented any new feature for a long time (not even fixing certain bugs).
http://konolige.com/dbignore/ exists for Mac OSX. It's .gitignore for Dropbox, but the instructions are a bit advanced.
Anyone care to figure it out and write simple step-by-step HowTo?