One of the success reasons of arduino is the factor that you can unplug it from any project and use it on another for research/tests, etc. That and the fact you dont really need to worry about learning a lot of electronics theory due to the existence of shields ( and also breakout boards for smd components) you can buy for the most favourite purposes ( games, lcd, midi, mp3, etc).
Allied to the fact that the IDE also cut short a lot of the work needed to program and compile a program(aka sketch), which allows even 12 years old to get the gist of it quite quick.
Plus the best of it all- ITS OPEN SOURCE !!!