Well, perhaps the first thing to note is that, whilst Unity Panel looks very similar to Gnome, the Unity Panel can't be customized1: what you see is what you get - whether you like it or not. The Gnome applets have been replaced by indicators and users can't add, remove or modify these tools in any way. One of the things that I've always loved about Ubuntu is that I can (given my limited talent) customize the interface to my heart's content; but Canonical seems to have decided to implement a one-size-fits-all policy. Of course, if you really must have a customizable panel, one way to avoid the Unity problem (and utilize Gnome in 11.04) is to select Ubuntu Classic from the Sessions options2 during start-up.
Whilst I'm not terribly impressed with the lack of flexibility in Unity's indicators, it's not all bad news! A really nice feature is the integration of (maximized) applications' title bars and main menus in the main Unity Panel.
This is a great space-saving device, especially for laptops and notebooks. Resized (that is, neither maximized nor minimized) applications resort to the usual application Title bar. However, not all applications seem to have been fully integrated and some programs (LibreOffice for instance) retain their menu bar even when maximized.
The Ubuntu Menu has also been removed from the top panel and replaced by Unity's Dash and Launcher (more of which in a later post). In look and feel this is reminiscent of the (now defunct) Ubuntu Netbook Remix interface and, whilst it looks great, I find that it makes navigation less intuitive.
Everything else is pretty much as it was with Gnome-Panel with some minor rearrangement of services3. However, one nice touch is the addition of a shortcut to the default media player in the sound indicator. The interface now includes some basic controls (skip forward/backward & play/pause) in the Unity panel drop-down.
All-in-all the changes are OK and integrate well with the other innovations in 11.04: Natty is well worth a test drive if you have a test machine lying around.
1 This is not quite true: Unity Panel's opacity can be modified using the Ubuntu Unity Plugin in the CompizConfig Settings Manager: but that's about it!
2 The Sessions options can be found on the bottom panel when your Greeter Window appears and requests your username and password. Use the spin control to select Ubuntu Classic and login in the usual way.
3 For instance, the shortcut to the Ubuntu One Control Panel has been moved from the session indicator to the communication indicator.