There's just something fun about upgrading. Everything's new and shiny. Aside from the official what's new, here's a few of my thoughts after using it for a day or so.
The UI Redesign is beautiful. I like the new icons. They have a modern feel, but are similar enough to the old ones that you can still find what you are looking for. Here's a taste:
Dynamic Text is a great replacement for my old VBA code that automatically updated text elements that showed the current date and path to the .mxd.
VBA is officially unsupported at 10. You can still use it but you have to install the VBA developer kit as well as request a license (free) from ESRI and then install it manually on you license server. It's totally gone with the next release. While I knew that this was coming, I'm still amazed at all of the VBA out there that's going to have to be ported to a .NET addin or the arcpy module. Thank goodness that I've got experience with both types of development.
You cannot administer an ArcGIS Server 9.3.1 instance with version 10 Desktop. This is the reason why I have a laptop on my desk running 9.3.1. :(
Basemap Layers are awesome. They give a cached map service feel to your basemap layers in ArcMap. They're not as smooth as I would like, but they are still way better than regular layers.
Most of the windows are dockable know which can easily be hidden and shown without a map refresh.
These are just a few of my first impressions. Anyone else out there running 10? What do you think of it?