To be honest, am I surprised? Not really. Fedora was also the first mainstream distribution that decided to have ext4 as its default filesystem back in 2009. But this time I think they really overdid themselves.
While Btrfs has certainly come a long way, as can be seen from their page they still consider it to be under heavy development. Although currently no disk format incompatibilities are expected in future versions, I think it's far from saying that it can be used securely in production environment (I'm not criticizing the developers of Btrfs, it's simply a process that takes time).
While it is certainly possible to select another file-system during the installation, usually the default file-system choice should be the 100% safe one, the one which will provide the best experience and stability to majority of people.
In case of ext4 nothing had gone wrong, but I think that this time they're really playing with fire. The problem here is that it's the inexperienced users that will be the ones to get burnt if something goes foul.
In the end, just in case some Fedora fans/devs read this post, I'm not trolling Fedora. I'm just making a somewhat stronger observation on the history of the project which has certainly been able to create some good momentum in other distributions as well (particularly I liked the fact that Fedora had included nouveau relatively early in its development, which caused some other distros to do the same), but I'm still very often left with a feeling that Fedora serves as a testing ground for Red Hat's enterprise solutions. And in the long run that is not a good thing.
Copyright (C) 2012 Branko Majic. Verbatim copying and distribution of this entire article are permitted worldwide, without royalty, in any medium, provided this notice is preserved. Code snippets found throughout the articles are licensed under GPLv3 or later.