In late December there was a public call for testing by the lead DNF developer, "planned Yum replacement now ready for user testing."
The DNF package manager originally premiered as a non-default option in Fedora 18 and has quickly advanced with its new features and speed.
It's also been working to meet parity with Yum by supporting plug-ins, an external API, and other features.
DNF isn't expected to replace Yum until at least Fedora 22 in late 2014, but Hawkey package management is expected for Fedora 21. DNF builds upon the Hawkey library.
While DNF isn't being forced upon users right now and Yum is still the default in Fedora 20 and Rawhide/21, some users aren't liking DNF at this point. There's a mailing list post today that begins with "[it] look like it starts to happen again: a replacement which is not ready."
The aforementioned post cites two other mailing list posts by different users citing that with DNF it's possible to erase all of your installed kernels quite easily and Yum plug-ins won't be directly supported by DNF. Thus to some DNF isn't a "drop in replacement" for not acting the same as yum with regards to its plug-in support or command handling. DNF though is, of course, still under heavy active development and is months away from reaching Fedora installations by default so I really wouldn't fault the project much at the moment when things are admitted to be not-yet-developed or broken.
In related DNF news, DNF 0.4.10 was released today. DNF 0.4.10 ships more bug-fixes plus some overdue command-line features and extends the plug-in support with two new plug-in hooks for providing new useful functionality.