Prosaic or Humorous Scenes
While fantasy illustration is no stranger to humor or protagonists that are less than competent, but not large than life, these sorts characters are depicted in a higher proportion of D&D art.
More Detail on Monsters
Monsters in much traditional (pre-D&D) fantasy illustration are best described as "phantasmagoric" or fanciful, charitably--and perhaps even outright goofy. D&D monsters are not always anatomically or realistically considered but they are generally detailed and usually dynamic.
Placing the Viewer with the Protagonists
The eye of the viewer is often positioned as if they might be a companion of the pictured protagonists or at least a close observer, rather than viewing the action at a remove. The primary focus then is often placed on the antagonist (or monster) rather than the heroes.
This one is obvious due to the "party" structure of rpgs, and it is perhaps the one most frequently supported by the art. The party is often displaying teamwork.
Anyway, I think those sort of make the point. I do think there are some others regarding costuming and composition of scenes, but these are the ones I feel most certain about. Of course, there is a lot D&D art that don't show these characteristics and there is some non-rpg fantasy illustration that does. These are really about tendencies, not absolutes.