Well, GNOME is my primary DE and I like it all the things considered.
My main criticism for the GNOME project is the very poor documentation which is also linked with customizability.
Why would I need an extension to change the color of panel, for example? Then, say there is a reasonable explanation for this. But, again why would I need to install an extension provided by third parties, god knows who? GNOME project tries to embrace these sort of extension (mainly because they need to NOT they want to since main customizability is very limited for the dekstop) so there is a kind of GNOME web extension feature, bla bla. But, when I install the extensions that way it is not good enough:
1- I have no idea what I am installing or whether that specific project will be continued or not. Sometimes, I can not find the same extension I installed couple of months later. What?!?
2- There should be an environment for extensions, controlled by GNOME project, but better than the current one. For example, I like that some extension projects are provided as git repositories so we can package them if we want to.
At last coming back to my first point, there should be a good documentation on how to write new extension when we want to. It should be in a way that I could understand simple basics in 10 mins or so since most of us dont have much time to deal with all that, it should be simple enough for everyone. This article kind of reflects my point, http://mathematicalcoffee.blogspot.com/2012/09/gnome-shell-extensions-getting-started.html.
I heard that GNOME plans to include GNOME tweak under their project. So, I believe that is good step for GNOME customization. And again, since Ubuntu is planning to (or already did ?) switch to GNOME desktop by default, there may be some attempts to address these issues.
As @joekamprad says, I really would like an optional tiling version available.