DNF 0.5.1's main feature is its less verbose with its text output during the dependency-resolving process. Up to this point it would spew dozens or even hundreds of lines of text about dependency processing. DNF 0.5.1 also now reports about bandwidth savings when using delta RPMs.
There's also a few bug-fixes to DNF 0.5.1. More details can be found via the GitHub-hosted release notes for this Yum successor that's currently available in Fedora 20 and may become the default around the Fedora 22 time-frame.