Simplicity.  Sometimes a simplistic concept can be an amazing one.  Take this game for example.  A Space Shooter for 2 Bucks! takes the shoot 'em up genre and doesn't try to clone geometry wars, doesn't try to make a next-gen Galaga, and doesn't try to innovate the genre with some fancy new gimmick.  It just took everything you would expect a shooter to be and put it into one pretty little package.  A $2 package that's worth every penny.