The biggest difference is the layout. The "home" screen has all of your most used buttons in one place so you aren't jumping back and forth between customer, vendor, banking screens, etc. Also, you can now modify the look of your invoice by adding or removing items you want on the layout such as address, phone number, etc. Very cool!
The one annoying thing from my perspective is that the general journal entry is now under a menu item rather than "out in the open." I do A LOT of gj entries!
I've only been working on it for 2 weeks but I think as I get used to it I will definitely prefer the '08 version.
BTW - my cpa told me they are "sunsetting" payroll support in QB 2005 so you will need to switch to '08 if you do QB payroll with their updates.