The default desktop task has been changed from GNOME to Xfce within the Tasksel Git. While one might assume the change is due to the criticism expressed by many GNOME users since the 3.x series -- with some saying it's losing its relevance on the Linux desktop -- the main reason expressed within the Git commit comes down to Xfce being able to fit entirely on the first Debian CD.
GNOME has grown in size along with other packages and it's become rather challenging to fit a GNOME desktop installation onto the first installation CD along with other necessary Debian Linux packages. Xfce, meanwhile, is much lighter. This means that Debian users with only the first CD could at least bring up a basic desktop environment rather than needing multiple CDs to get going. "This ensures that the desktop will fit on CD#1, which gnome currently does not."
Fitting an entire desktop Linux distribution onto one CD has become a challenge not only for Debian but also for Ubuntu and other Linux distributions as well. Some have now switched to DVD ISOs by default, aiming for a 1GB USB image target, and/or other creative tricks to fit as much as possible onto a single CD. If you want all of the Debian Wheezy install archive though, it will take 73 CDs or 11 DVDs.
Additionally said in the Git commit making the desktop change, "There may be other reasons to prefer xfce as the default as well, but that is a complex and subjective topic. Unfortunatly, Debian does not have a well-defined procedure for making such choices, though it certianly has well-defined procedures for reviewing them. So, I've decided to be bold, and continue the tradition of making an arbitrary desktop selection for Debian in tasksel."
GNOME (along with KDE, LXDE, and other desktop options) will continue to be available through the Debian archive but this is just a change about the default desktop selection.
For more details on Debian 7.0 Wheezy plans (and Debian 8.0 Jessie), see my notes from DebConf Nicaragua that happened in early July.