The D3100's body is a little smaller and lighter than the D3000's--it's the lightest in its class--but Nikon has also managed to cram more direct-access controls onto it. It's quite a comfortable camera to shoot with, though it feels a little plasticky.
Nikon offers the same set of mode options as with most entry-level dSLRs, though Nikon goes a step further with its help system, offering a Guide mode that walks you through different types of shooting. There's Easy operation, which, like Auto, provides access to a limited number of options, as well as an Advanced mode (the second two screens), which describes the appropriate settings for the chosen scenario and then allows you to change the settings yourself. As with the D3000, my one quibble with this is that the controls don't always function the same in this mode as when shooting normally; so, for example, here you'd adjust shutter speed with the up/down buttons on the multiselector, while you'd normally use the command dial to change the speed. This might confuse some people.