From my experience with it, it's the easiest to manage environment I have been in. I've used nt4 and 2000 and 2003 makes things just a bit easier. Once you start thinking central management, nt4 just doesn't cut it any more. Plus getting things working in AD makes life that much better, and the enhancements that 2003 has over 2000 shows through when working in AD. The ability to update and deploy GPO's to groups or individuals withouth needing to manage scripts to drive it is a nice plus.
What I don't know is why you don't want to move away from NT4?