What you get:
-FULL customization. This may seem crazy but you get WAY MORE oportunities to customize your Nexus One. Although the Nexus One is already very customizable, if you root you can have custom icons, battery indicators, custom messages on the lock screen, custom status bar colors, transparent menus and etc.

-720p TRUE HD. You are able to record in 720p with 25-27 FPS which is absolutely beautiful. You have the chance to flash a camera and you will be able to TOUCH-2-Focus, something the N1 doesn't have in general.

-SPEED. The speed is phenomenal. You are able to over clock your phone past it's said performance of 1GHz.

-Battery. THe battery life is doubled if not TRIPLED. I caught myself charging my N1 as often as I did with my iPhone. AFter Rooting and installing Cyanogen Mod 6, I don't charge my phone throughout the date.

-if you root and install Cyanogen it's no different at face value than FroYo. So it's not like you would be upset at CHANGE. YOu have a HD camera, a separate menu for Cyanogen Settings and just overall improvement with the N1.

You are able to flash THEMES made over at XDA. Themes include CUSTOM EVERYTHING. From icons, to status bar, to charging notification, to text. It's much better, I promise.