What are the things you feel that you should have known before your first postdoc position?
Answering the question in the body (about you) rather than the one in the title (about us, it seems?):

Work on developing your own research agenda. Try to move away from the work model where your advisor tells you what to do and you do it, which is often (although not always) the modus operandi during Ph.D.; and towards asking your own questions, developing and honing your own taste of topics/techniques/areas, mapping in your mind the important and promising directions in your field, etc. 

Obviously, in doing so, clear communication with the advisor about their expectations and your goals is essential. E.g. some people like to keep their postdocs on a shorter leash than others, or may have specific obligations to the funding body regarding your research.