The platform uses the "Liftarm" beam construction style - all pins and no lugs.
The NXT brick has a dot matrix LCD display, and can play simple audio files.
It has ports for four digital sensors. the kit comes with Sonar, Light, sound and touch sensors. There are additional third party sensors like colour sensors, compass sensors and others.
It has three output ports, and comes with three active servo motors which have position feedback, and can be used as rotation sensors too.
In terms of programming the NXT it comes with a visual "g" language based on Labview. However, Lego embraced the hacker community which grew around the RCX, and encouraged many other languages to be developed, so C compilers and so on are easily available.
Backwards compatibility with the RCX comes in the form of adaptor cables available on the Lego website.