Beat Hazard (along with the DLC) is one of the few games that I fully appreciate. It's definitely up there as far as quality twin-stick shooters go. There are just so many things that make this game good.
Play your own music - This was the very first 'play your own music' game that I got on Steam. It's not the most special feature on Beat Hazard but it's the main reason I enjoyed it as a twin-stick shooter. I don't have a lot of experience in the genre so being able to play on my own pace depending on the song used was a major positive factor for me.
Difficulty Adjustment - The difficulty is easily adjustable in a multitude of ways. You have your conventional difficulty settings (easy, etc, etc). Then you have visual intensity which to me is somewhat of a visual handicap (with benefits of course). Two other ways to adjust difficulty would be by picking songs with varied intensities or by playing with the new DLC content which adds ship customization complete with stats. In a game with this many options, it won't matter what skill level you are within the genre. It's still very possible to feel a small sense of accomplishment even as a casual player.
Music Discovery - In this day and age, I no longer listen to the radio unless I'm really bored while driving from location A to B. Beat Hazard allows you to play while listening to various radio stations (custom or default added). In all honesty, I have discovered a ♥♥♥♥ ton of music during the few months that I have been really into Beat Hazard.
There's a lot more things that make Beat Hazard a wonderful game. For the sake of simplicity, I merely listed 3 of my favorite reasons.
It's nearly impossible to get bored in this game.