Low amount of pixels and colors. Sometimes varying resolutions. Flat shading. Dull colors in certain/most scenes. That's all I can get out of it for now, there's probably more.
Generally this style is easy to achieve if you're skilled enough (I only consider myself decent and most of my small mindless doodles do tend to resemble this) but if you're planning to create
all of your art in this style simply because you're in love with it, that's probably not a good thing to do. Be warned, it might prohibit you from growing as an artist, and you'll never be able to make things like
this (more at the Pixeljoint Hall of Fame if you want to check that out). Grow enough as an artist and things like this (no, not the dragon) will be a piece of cake.
Doing studies, searching around for advice, taking notes, drawing from reference and etcetera may be tedious, but is usually the best way to go. Or grow. Either word is okay. If you're afraid of doing that, sorry to scare you away, you probably won't see much progress. If you want to stay the way you are, that's fine. I'm just telling you something you might (not) want to hear.