Steve Winter, who was involved with D&D since its early days, wrote an interesting article at the Howling Tower. I agree with this article. Dungeons are one of the most controllable environments from the DM point of view. The players can't wander off-map as there are physical boundaries and each room is quite clearly filled with challenges and monsters that won't change from session to session (well, they can but that's another story). If you take a look at the very first modules published by TSR (In Search of the Unknown, Descent into the Depths of the Earth, Steading of the Hill Giant Rift and so forth), you notice that they were all originally designed as tournament modules for GenCon. Thus, they had to be manageable and limited in scope given the limited time of the event.
I am also a firm adherent of the notion that the word "dungeon", with a strong association with underground places, can also be applied to other locales like castles, forests, ruins, buildings, etc. In this case, the word itself goes beyond its pure semantics and becomes an umbrella term for every constricted place that allows the players to interact with the environment during their adventures. I will relay my thoughts on this subject on a future post. For now, enjoy the article.