What makes a D&D session different from a DM running a bullshit session, where s/he interacts with the players a-la story teller, which typically happens before and after actual adventure? The answer is - DM's Preparation. Arguably, there are places outside game mechanics in fantasy role-playing. For instance, the strategy of the Orc Chieftan opposing the players with his band of 60 Orcs. One the simplest level, it is easy to use up 60 Orcs in Random and Room encounters, but if you decide that the Chieftan leads his warriors intelligently, you will have to outline the Chieftan's strategy and general reactions in most predictable contingencies. You have to commit this to writing, much like the layout of ships in a game of Battleship. Why must you do that? You must do this to give the players a chance to actually defeat their intelligent opponent. You see, without this written down plan, the conflict will be between the players and the DM, in which case the DM is the Omnipotent adversary. If you have a contingency play for the Chieftan written on paper, you can actually show it to the players after the game in a moment of discovery for the players.
How is the D&D game different from a modern day Node-Scematic adventure game? Node based adventure design, as developed by Justin Alexander of the The Alexandrian fame, is very useful for modeling complex event based adventures. Dungeon Crawl adventure is a stylized form of story presentation in the game. Wilderness Hex crawl is another related story-telling form. But beyond that, there are complex non-site and not geography based adventures. For instance, a quest, that requires players to solve a mystery, find a missing sage, influence a King, deal with treacherous competition while planning for an expedition, that, as last will be a wilderness and a site-based dungeon adventure. You can't map this kind of an adventure geographically. You have to map out social networks of individuals, locations, where social encounters take place, and clues, red herrings, ets. Think of it as a dungeon map, except that each "room" is an encounter with a specific event that moves the story forward. These events may be separated by walls and miles. To go from Event A to Event B may be a simple walk across town or a three week journey with a wagon train.
What separates D&D from those other, later, games, is that later games concern themselves with specific events, and assume that you will get safely from Event A to Event B, where you will be risking your neck. D&D assumes a tactical navigation between the Event A and Event B. If the players walk across the city, you will roll for encounters and unusual events, as the players walk across the city. If played correctly, these encounters add the sense of place to your campaign, butg they have nothing to do with the story, through which your players are playing, which will only move forward when they reach Event B.
Why is this important? Because in a Sandbox campaign, and all OSR's aim at Sandboxes as opposed to meta-plot gaming, players can literally go anywhere and do anything. Ideally, all of the geographic and social directions, in which players might go, have been already considered and mapped by the DM. The problems begin, when the players cross into the uncharted territory, for which the DM has no encounter table or conceptualization. At which point, unless the DM stops the session, s/he starts improvising and the bullshit session will begin. A closely related situation, is where the players are lost in a Sandbox and are deciding, where to go next. So far as the DM has the mental framework of what lays to the north of Kingdom A, DM can improvise with rumors and tales of the faraway places. If the DM is worth his salt, that information will be passed via the loose tongues of the NPC's whom the players engage in conversation. Once the players decide to move out or they focus their attention on a place for which the DM hadn't dreamt up anything yet, DM should stop improvising and end the session.
Another example of it are players wanting to build a fort, dam up the river to flood the dungeon or some other such scheme that will take them outside the DM's box. Typically DM shows limitation of his or her spirit, by telling them why such an action can not be done. A cool DM would stop the session, and research the plan of action that the players want to take. Whether timber palisade construction or earthmoving, DM should figure out if an how it can be done, what will it take, who has the knowledge, how much it will cost, etc. And then, during the next session, he can let the players try to figure it out how to do it and try to pull it off.
A DM is like a river. If he tells the players NO! they have touched the river bottom. This is where railroading and linear adventure, and the players dissatisfaction with these elements, begin. On the other hand, if the DM is prepared, and he knows the material better than the players, and is one step ahead of them, players go in over their heads and are immersed in the DM's game world. This is where magic happens.