I am a map lover based in Southern California. Having had the frustration of dealing with commercial GPS maps and their inadequacies, I am thrilled that an open source alternative exists.
In terms of what I map on OSM, that depends on my mood. Sometimes I like to target a specific area and detail the heck out of it, other times I find a particular feature of interest (e.g. chimneys, power plants, airports, etc.) and add missing points around the globe. An ongoing mapping passion is railroad lines as they are often vaguely roughed in by TIGER maps or are missing completely, especially spurs and branch lines.