I have 12.04 installed but I am testing Xubuntu.
You need to understand that Ubuntu is based on Gnome. Gnome3 is what Gnome is putting out so Ubuntu uses Gnome3. Unity is a plugin for Compiz, developed by Ubuntu to be the default desktop environment.
Gnome Shell, which was around when 10.04 was in testing, is the default DE for Gnome3.
You will be able to install Gnome Shell in Ubuntu 12.04. Many people on the Ubuntu+1 "testing" forum are using GS and seem to like it quite well.
The problem is that there are few testers, many "improvements" to Unity and the switch from Gtk2 to Gtk3 on top of everything else. I would not encourage anyone to upgrade until the release of 12.04.1 (first step release of the new LTS) which is scheduled for July 19th. They should have things pretty stable by then.
If your hardware will not support Unity there is Unity 2D which will automatically take over on your box. Some folks are using it and prefer it to Unity 3D.
Gnome Shell will have to be installed from the repo and then chosen at the login screen (session selector).
If you want to stick with a panel type DE I would check out Xubuntu 12.04 which is much more stable then Ubuntu 12.04 at this point.
Part of the reason is that Xfce4.8 is still using gtk2 and won't go to gtk3 until 4.10 is released. I think 4.10 will be in 12.10.
As a long time Gnome user I prefer the Xfce4.8 DE to the Gnome panels you are using now. I would not switch back to Gnome if they reinstated their panel DE.
Rhythmbox, as you say, is the default music player for Ubuntu 12.04. Banshee is just too buggy and they have had a lot of complaints.