Why can't I upload videos to DaCast, but used to be able to in early 2013?

Yes, in June of 2013 DaCast improved our upload system which changed a few things, notably it means that uploading is done over port 8333.

So if you are having trouble uploading (either as a new client or someone who was able to do so before) make sure your firewall or router is enabled for port 8333.