I started using 2004 recently. It takes a bit getting used to the new menu format, I'm not sure if it's an improvement, just a different way of displaying the icons. I am amazed how slow I am compared to my custom icon layout in 2003 (the same one I've been using for 3 years), not to mention the massive amount of space the command center takes.
The one thing that I absolutely hate is how the feature tree propagates to the graphics area when mating parts; I find it difficult to select the surfaces for mating thru the words on the feature tree. Is there a way to disable the feature tree from propagating to the graphics area when mating parts?