There surely could be some improvement for desktop usage, but I think that would necessarily take away from debian's focus on the stable release and being universal. For example, snapshots would dictate a filesystem choice and ubuntu supports fewer architectures.
That said, what you are proposing sounds very much like tanglu (roughly, do what ubuntu did but stay debian compatible).
It may be easier to base it off of Linux Mint Debian edition. However there is a new distro I think the community may be able to migrate to. Tanglu it is related to Debian but tries new technologies and has a six month release cycle.
Testing and Unstable are quite painful without apt-listbugs and keeping an eye on the packages apt says it will remove. But I've not really run into any problems updating every few days. However I learnt a lot about how to break my installation during the gnome 3.12 to 3.14 transition. I'm going to see how Gnome 3.16 progresses once the freeze is over, but I might check out Tanglu again.
He probably wants something like Debian CUT (Constantly Usable Testing) but I don't think CUT ever gained much traction and is even a thing any more. Ubuntu or Mint release like this, but they make changes to Debian that I don't care for and it would be nice to have a vanilla Debian that released like this.
Debian Tanglu seems to be a current distro that is aiming for this goal.
Are your issues with Ubuntu/Canonical about Unity or more philosophical?
If not philosophical -
You could always do an Ubuntu Server install (which includes no DE), then add the DE/WM/etc of your choice.
I suggest this not because I think it's a better choice than the others you listed, but because it keeps you in an environment you are already comfortable in admin-wise, hits your points about support, and "just work", and allows you to not have to grapple with replacing Unity/KDE/LXDE/Gnome if you want to try something less mainstream and/or not covered by the other "flavors" of Ubuntu. (There's also a Mate version in the works if that's your cup of tea...)
If your issues are philosophical, this won't solve them, but otherwise I think it might be worth considering...
Edit: The next time I get the itch, I'm leaning towards Tanglu. IIRC they have a unique approach which is Debian based, and also provides access to fresh packages without having to run (only) Sid. I seem to remember that they pull from the various debian repos as needed to provide freshness + general stability, but it's been awhile since I looked at them.. http://tanglu.org
I have been using Arch Linux for years at home and OpenSuse 13.1 at work for couple months. Arch Linux really is what you want it to be and because I'm happy with Gnome's 'stock' apps everything integrates pretty well. You can even use gnome software centre to install/remove apps.
OpenSuse works too but altought they both have almost same Gnome version, some shell extensions dont install succesfully for Opensuse.. Yast is handy, but breaks system look because Gnome also has its own Settings app. It would be nice to see yast integrated in same place with gnome settings.
This is interesting new distro based on Debian Testing and looks good with Gnome. If I have time, I would like to get it running on my work laptop: http://tanglu.org/
There is a new distro that will be based off of Debian testing and will continue to update after the testing freeze and it integrates with systemd by default. It may not be ready for you yet but may be worth a shot since you are already comfortable with Debian. The new distro is called Tanglu