Like many in its price class, the D5100 feels plasticky, but solid. One of the design changes over the D5000 is the more prominent slope on the left shoulder which I'm not crazy about--I think it makes the camera look lopsided--but it really doesn't affect the shooting experience. Like the D7000, the conveniently reachable Fn button sits below the flash button.
The back controls are laid out in a typical fashion. The information edit button--not to be confused with the info button on the top--brings up the interactive information display where you adjust most of your shooting settings. My only gripe: there's no way to lock the navigation switch. Since I shoot in single-point area AF mode, I frequently moved the AF point by accidentally pressing the switch.
In addition to bumping it to a larger, higher-resolution display, Nikon changed the movement of the articulated LCD from drop-down-and-twist to a more traditional flip-out-and-twist.
Although it's set up differently than the D7000, the D5100 also has a great layout for the occasional video shooter. The switch on the side of the mode dial toggles between regular and Live View/Video mode, so you don't have to use an awkwardly placed mode on the dial. And the record button is in a great spot by the shutter; it's easily reachable with your forefinger, but not in a spot where you're likely to hit it by accident.
For the D5100, Nikon added Effects to the mode dial. Your choices are Night Vision, Color Sketch, Miniature, High or Low Key (which really belong in the exposure presets), Selective Color, and Silhouette. All operate in movie capture as well as still.