Body elements

The most immediately apparent change to the D3100 is the addition of the liveview lever and movie record button on the rear of the camera. They're well positioned so that they can be easily activated, but with little risk of doing so accidentally.
The D3100 gains two ports over its predecessor. In addition to the USB and AV sockets, the newly video-capable camera gains an HDMI connector.

In keeping with the rest of the company's range the D3100 gains a connector for the GP1 GPS unit, which doubles as a socket for the MC-DC2 remote control that's shared with the D5000 and D90. Note though that the wireless remote receiver has disappeared.
The D3100 adds a drive mode switch, offering single, continuous, self timer, and Nikon's now-standard 'Quiet' mode. This is a nice touch, making for one less thing that requires setting in the 'info' screen.

The D3100 can utilize SD, SDHC or SDXC cards. It's worth getting hold of some fairly large, fast cards if you're hoping to make any great use of the camera's 1080p movie recording capabilities.
As with all the recent little Nikons, the D3100 doesn't have a focus motor built into the body. This means it's unable to autofocus with older-design (although still currently available) 'screw-drive' lenses, although these can still be used in manual focus mode. Fortunately most popular entry-level lenses are now available with built-in focus motors.
The D3100 uses a new battery, the EN-EL14. It's a very similar capacity to the one used in the D3000 but offers a slightly higher voltage and recessed, anti-shorting connectors. As a result it's entirely incompatible with the older units.

One slight disappointment with the D3100 specification is the rear screen, which looks the same as the D3000's. It's a large panel (3.0") that's bright and contrasty, making it about as usable in bright light as these things get.

However it's a little low in resolution compared to many screens we've seen on recent cameras. It's by no means bad at 230,000 dots, but it doesn't quite live up to the rest of the camera's specification.