I’ve always found gnome desktops to be something of a hassle to find good themes for. Usually it takes a solid amount of time rooting around in one of the repositories, checking compatibility, etc. etc.
One of the places I’ve found that has a reliable theme pack that’ll install and give you a clean looking desktop to work with is the Bisigi Project. Just pop their repositories in your system, install, and go.
Gnome 3 support is coming with the release of Ubuntu 12.04. For now you can hack it in for previous versions of gnome or get it working with LXDE.
Ever wanted audio, text, or video to open up in a different application than whatever Debian has slotted for your default? Want to get down to the nuts and bolts and do it manually?
Well then. Take a look at /etc/gnome/defaults.list for what application is used to open up what extension or mime-type, and bring up the /usr/share/applications/ directory for the configuration files that dictate how that application is going to open up.
For instance, I changed .pls and .mp3 from Totem (possibly the last media application installed) over to audacious2-gtkui for better integration with the look and feel of the gnome desktop.