Love the hyper lightweight system powering the games. Very reminiscent of OSR and AncientSchool mechanics,  but without being a clone (and carrying the burden/assumptions about style of play). Honestly, I would be happy with the system alone, but the series goes beyond that!

Each game gives you new options and populates the game world I am a big fan of those "gestalt settings" created not by directly stating the setting elements, but by inclusion of specific rules and random tables entries  that paint the picture of a setting which your group can add to during play.

Another awesome idea 2400 introduces (and I hope others will follow) is modularity. You can play a pretty focused game using any of the booklets, or you can mix-and-match to create your own version of 2400 universe!

In short, love the system, love the modularity and love Beeple's art. It's a slam dunk for me!