In a way there are two competing definitions of "depth" in popular usage -- one involving the amount of content (and maybe customizability) a game has; the other, the extent to which gameplay mechanics reward practice/mastery, exhibit emergent behavior, allow for things the designers didn't seem to expect, or otherwise offer more than first meets the eye.
Neither definition is wrong, really; it's just a question of where your priorities lie. I suppose neither form of depth means much if you don't "click" with a game, since neither loads of content nor well-thought-out mechanics matter much if you don't find yourself compelled to spend time with a game.
Anyway, nice blog and nice project! Glad to know of another person aiming to "beat 'em all", and to have another prospective contributor to the NES thread.