If WebKitGTK+ applications hang or become unusably slow when an HTML5 video is supposed to be, you might be hitting this bug.
If Totem crashes on startup, it's likely this problem, reported against cogl for now.
Feel free to add a comment if you see other bugs related to DRI3, or have more information about those.
Update: Wayland is already perfect, and doesn't use DRI3. The "DRI2" structures in Mesa are just that, structures. With Wayland, the DRI2 protocol isn't actually used.