Bastien Nocera, a Red Hat employee and Fedora contributor, wrote a blog post today entitled Watch out for DRI3 regressions. Bastien says that DRI3 has necessary fixes for X.Org and Wayland, but watch out as "it's still young in its integration."
There's a few bugs that users are hitting with the Intel DRI3 stack that includes WebKitGTK+ applications hanging or becoming unusuably slow, Totem crashing, and other potential issues related to this latest revision to the Direct Rendering Infrastructure 3.
At least we're still a few months out from the official release of Fedora 21 so all early fallout should be addressed in time. DRI3 adds support for passing file descriptors via DMA-BUF, client-allocated per-window buffers, and other new features over DRI2. Besides Intel, the other open-source drivers are also working towards DRI3 support.