The commercial transport database seen in TNG: "Inheritance" suggests that travelers that required a special class N environment also often required low-gravity or even null-gravity accommodations. It was also the only planetary class mentioned to require special accommodations in the relatively extensive sample of Federation travel movements, perhaps suggesting that among planet classes with special environments it was among those more likely to produce intelligent species.
Information taken primarily from Star Trek: Star Charts indicates this is a "reducing" planet having an age that ranges from three to ten billion years, and a diameter between 10,000 and 15,000 kilometers. Star Charts mentions reducing planets are located within the "ecosphere" of a star system, with class N planets categorized by a high surface temperature due to greenhouse effect (which causes all water on the planet to exist only as vapor) and an extremely dense atmosphere, comprised of carbon dioxide and sulfides. If life can exist on a class N planet, it is unknown in what form(s) it may appear, a textbook example of a class N planet is Venus.