In short, what makes a good version control system "good?"

The things that I (and others) stuff into it should come out in exactly the same way. Byte by byte, bit by bit. (SVN has some munging for newlines on different OS' - thinks like that are OK if they are optional. Likewise for ID strings.). Sadly quite many version control systems don't adhere to that rule.

The rest are rules that mostly apply to all other programs as well: they should be fast, use little other resources, are easy to understand, easy to use, modular to use, easy to integrate, flexible etc.