The Exchange Team has changed their recommendation around the disk space required to install Exchange 2013, to be in line with the way 2013 works.
There is a lot of logging going on in an Exchange 2013 server, which needs a lot of disk space. Hence the minimum disk space required for installing a 2013 server is 30GB. You can have 2013 installed on the C drive or a dedicated drive for the Exchange binaries, as long as you have a minimum of 30GB.
Above is the slide from one of the TechED NA Exchange session.