Gnome3 is bloated and slow compared to, say, XFCE which does the same, does more, is more reliable, fast and consumes much less resources
XFCE may use slightly less resources then Gnome right off the bat, but since Gnome makes more advantage of shared libraries and you need more 3rd party applications to reach feature parity with XFCE then the difference is mostly a wash. The rest can be easily contributed to the placebo effect.
The real source of the perception of Gnome being 'heavy' is because that the desktop configurations that systems like Ubuntu and Fedora choose to use tend to use more resources. Which is not Gnome's fault it is just how distributions choose to ship it.
If you want something that is actually 'lite' and based on GTK then the only real choice is LXDE and by using that you are giving up a LOT in functionality. If the functionality doesn't matter to you then it doesn't matter and it's a actually nice desktop.
I really doubt most people in this thread ever actually used Gnome 3 in a meaningful way. The only distribution shipping a decent configuration out of the box right now is Fedora and unless you have tried using Fedora 16 or 17 for more then a week or spent time conjoling a decent configuration out of Debian or Ubuntu then you really have no idea what you are talking about.
This is why I am excited about 'Gnome OS'. For Linux to really shine as a desktop OS it needs to abandon the 'do everything' nature of traditional Linux distributions and just concentrate exclusively on being a desktop. Effective virtualization has killed the need for a 'be everything, do everything' OS and desktop virtualization support is going to be built into Gnome.