Well, after playing with both the amd64 Ubuntu-based GNOME version and the Debian version, a few things are noticeable after a short tinker:
1. Mint Debian is faster. Sure it's not Arch vs Ubuntu, but it's definitely snappier, at least in a VM. Mint 9 shuts down faster but boots up slower. I restarted the two (Debian second) and I was back at the desktop faster on the Debian version even though it took longer to shut down and didn't have auto-login. Firefox also started 50-100% faster (subsequent starts were much the same between the two editions), but the versions are different so it's not a scientific test.
2. The installer is less "polished" but still much better than the Lenny GUI installer. I actually like using gparted, because it's simply faster than Ubiquity to do everything since no actions are actually applied until you finish setting up the task queue, and that means no rescanning of drives after every change.
3. No compiz as far as I can tell. I have VB Guest Additions on both and didn't get any fancy effects on Mint Debian.
4. No annoying Metacity "fixes" inherited from Ubuntu!!!! That means I can close a maximised window from the extreme corner, something that has bugged me to no end with Mint 9 and Ubuntu Lucid.
5. Get used to your optical drive being referred to as "cdrom" instead of "<disk-label>" when mounted in /media.
6. Missing a number of Ubuntu-specific apps. But you'd expect that. That means no jockey-gtk either
7. RAM usage is lower. I can't say how much lower exactly because I was comparing an i386 and amd64 edition so it's not a fair comparison. MDE uses about 115MB though.
8. Give me a break, I haven't used it enough to come up with 8 thoughts yet