For instance, GNOME 3.6 depends on a still-not-released version of ibus (a risky move for those who need alternate input methods). It also depended on gstreamer-1.0 which was only released 2 days before GNOME 3.6 and was not API-compatible with gstreamer-0.10 which had been used for years.
Ubuntu's integration work does shield users from certain controversial GNOME decisions. Examples include continuing to offer an easy way to change what happens when you close your laptop lid, allowing you to right-click on the desktop to change the wallpaper (this is part of having nautilus continue to draw desktop icons), and being able to change the theme. Those kinds of patches take time to keep working.
A bit off topic, but it is interesting that Fedora 18 won't be released until 2013. That's about 3 months after Ubuntu 12.10 was released. (I think a lot of that is because of their complex installer rewrite but still.)