I'm producing a chillstep song, and the research I've done showed me:
1- Structure wise, it's the same as dubstep (the old-school one), with tempo ranging from 130-140BPM.
2- There's a tendency of using airy pads, to create that ambient feeling (as elevate said, with lots of
reverb)
3- The bass is not too prominent. You'll have the sound, but it won't be the main sound playing
4- Normally they're composed using major scales (to achieve a chill sound, instead of a more moody sound of a minor scale), but this is up to you. For example, in my song, I start with a major scale, but after the breakdown, I change to a minor scale (and so, the song gets darker, melancholic)
5- Drums are not "in your face", so you might want to lower the velocity of the hits.
6- Feel free to use field recordings, to accentuate the ambient feel to the music. I'm using a sample from a recording of a waterfall at the first part of the song, and on the second part (the darker one) I used a recording of a thunderstorm. It's up to you.