To be honest, I've always thought of Wolfenstein and Grosse as being elaborate easter eggs rather than actual levels. I'm always disappointed in any game to find a secret level which is just a secret level, with nothing really special about it (E1M9 is a good example of this.) I enjoy secret levels that have an interesting twist, a much higher challenge level or something (an enemy, an item, a theme) which isn't seen anywhere else in the game. Gimmicks (like Commander Keen) are OK but not essential. I like secret levels to be unusual in some way.