The perfection of the formula. The graphics are beautiful for their time in the 2000s, the music is haunting, and you are given tons of spiffy new gear to play with in the form of iconic side weapons such as the Piranha and the Homing Laser, making it infinitely more replayable than the 2nd game, which was already a major improvement over the first.

The only problem I can think of is that SD4 couldn't find a way to fundamentally make the game better because SD3 is already really good. I think SD4 would greatly benefit from giving you more freedom in movement, but it's Star Defender, so I guess that's the best they can do.