I cant believe this feature doesn't exist in Bittorrent Sync yet. It is the only thing holding my company back from exclusively using it and is by far the most important feature needed for this kind of product in a company like ours. We have several large projects each several GB, without this feature everyone in the company would have to download then entire project directory. I am contemplating writing a small program to populate the .syncignore file with a simple tick box interface, which is an uber sucky solution to an even more uber sucky problem. PLEASE FIX.