I only really used the first DB for a little while, but there are some major differences that I could clearly see. First off the Interface is in a window now (and I like that cuz I like using multiple programs at the same time for my projects), there are new way cooler special effects you can do to object (box and sphere mapping, bump mapping, light mapping, etc...), BSP support, height mapping, built in multiplayer support, seems to run faster (especially collision detection!). There's a lot more, but It's 7:43pm my time and I did too much work, so I'm tired and can't think
Hope that helps!
1.00 GHZ processor, 256 MB RAM, GeForce 3 64MB, SB Live!, 8 cans of soda per day