It would be very nice to have an option for checking the remaining disk space from a remote node (and don't transfer any files if the value is lower than a certain limit). This way, I wouldn't have to transfer a whole bunch of files just to get an error in the middle of the transfer because the remote side has run out of disk space. If there would be such an option, rsync could just refuse to transfer any files but die with an error message instead. I guess this option will only be possible if the remote side is an rsync daemon but. Thank you for writing such a great software as rsync, I'm using a lot and am totally happy with it!
There is no need to build the check into rsync. The disk space check can be done in the --rsync-path (for rsync over a remote shell) or the pre-xfer exec command (for a daemon); if either of those fails, the transfer will be aborted. Unfortunately, returning a meaningful error message to the client is nontrivial. To do that, you need my deny-rsync script from bug 3945 comment 2 for remote shell or an implementation of bug 4320 for a daemon.