KidSnide
Adventurer
b) re: dungeon as adventure. At its core, a dungeon is simply a group of rooms filled with monsters and treasure. It could theoretically be one long tunnel with rooms for encounters and a boss at the end. Whether a PC goes "left" or "right" in a maze becomes largely irrelevant, as most players are smart enough to "follow the wall" until they find what they're looking for. A dungeon as an adventure is little different than a wilderness adventure (in a dungeon, your choices of direction are more limited, but the end result of getting from 'A' to 'boss' is still the same), save for the actual setting itself.
It's certainly true that plenty of dungeons (and adventures) are written as you describe. But you don't have to write dungeons that way. Ultimately, a good dungeon (or adventure) will have multiple routes or methods to reach the main goal. Whether the choice is between literal routes (e.g. heading straight through the main gate to the inner sanctum vs heading through the lower caverns to come up through the drainage gate) or different strategies (e.g. fight vs negotiate), a good adventure allows the PCs to make meaningful strategic decisions regarding how they complete their goals.
Furthermore, a well written adventure doesn't force the PCs to fight their way through every possible encounter in order to maximize their reward. In a good adventure, there should be a distinction between high-value encounters and low-value encounters and the PCs should benefit (and not be penalized!) for seeking out the high-value encounters and avoiding the low-value encounters. (The WotC 4e modules and the standard xp structure are horrible violators of this second point.)
-KS