- The lack of healing
- The lack of character class choice
- The loss of tactical options with magic gone
- It would suck
My first thought when I read this was that it would still be really fun. I have always been enchanted by simplified and pared down rule sets. I have been wanting to run an entire campaign using only the Pathfinder Beginner Box set and some monster books. I was intrigued a few years ago by a discussion about playing an entire D&D campaign using only the Homes set (a quick google search turned up nothing, but it happened). I find E6, the pared down version of the d20 SRD that only goes up to level 6, to be a very cool idea. I have always wanted to run a Castles &Crusades campaign using only their White Box set.
These kinds of simplified systems are especially appealing to me for megadungeons because I find megadungeon play to be pretty much the gamiest of gamey modes of RPG. You are literally going down to grab points and come back up. Obviously there are mysteries to solve, and interactions between the dungeon and the outside world to deal with, but it is simple at its heart. A megadungeon makes very little sense, usually not even within the already fantastical universes they tend to be set in, it exists to be a platform for the game. Since they are so game oriented, I want play to consist of easily implemented game loops. I draw a lot of inspiration for the way I run megadungeons from roguelikes (enjoy that rabbit hole).
When I run Stonehell, I make changes to Castles & Crusades to better suit the dungeon. In combat I use party initiative, and treat groups of enemies like buckets of hit points instead of individuals, damage is applied to the enemy in a group with the lowest hit point total and overflow damages the next monster in the group. I also use phases in combat, Missile, Melee, and Magic. Each side completes each phase before moving on to the next one. Since I use 1 minute rounds, we only have to worry about movement if someone is trying to do something special. This makes combat go very quickly, but it does remove a lot of the tactical elements. This makes for a short game loop for combat, and reinforces that combat is just part of resource and risk management, the actual challenge in the dungeon. I also use a dungeon exploration turn, exploration is highly structured, checking off torches, ticking off time etc. to move things along and reinforce the main purpose of the game, resource and risk management. I also use a very rudimentary town turn, one I am beefing up by adapting some ideas from Pathfinder's Ultimate Campaign book.
So, I obviously do not think it would suck. I would also not worry about the loss of tactical options in combat, because I don't think that is what megadungeon play is about. The lack of healing is something that you have to be more careful about, while I don't worry about it too much in terms of "the dungeon day"(see why below), it is a bigger concern in that certain monsters may be designed with healing in mind. If you are going to eliminate magical healing, you need to examine the challenges in your dungeon on the individual level a bit. The loss of character class choice can be an issue, but I believe this can be dealt with by making sure the characters are changed in interesting ways by the dungeon (more on this later).
In his response in The Clash of Spear on Shield, Chris C brings up Searchers of the Unknown, a slimmed down D&D variant where all the rules fit on the character sheet. In SotU there are no classes and, as Chris explains in his post, armor plays a large role in your character abilities. While I have never run a megadungeon campaign using SotU, I have run several games using the system, and I have some ideas on how I would use it in campaign play.
Character customization and differentiation will quickly become an issue in a long term SotU campaign. As Chris points out in his post, magic items, especially of the wondrous nature, will become highly prized. Not only are they rare in a low magic world, but they are a chance to customize your character and add options. SotU also treats scrolls a bit like a roguelike, everyone can use them and they are the only way to cast spells. The standard megadungeon/Underdark tropes also offer an opportunity to customize the characters, by actually changing the character. In a game with only one character class, fountains, potions, pools, altars, statues, and fungi offer the possibility of giving your character some permanent unique ability. As the characters progress through the dungeon, their abilities should be diverging as they gain different unique items, get cursed, receive boons from altars, and powers from drinking from fountains. Of course you will have to make sure that players are willing to engage the special dungeon items more than usual.
Rob Conley, from The Bat in the Attic, also gave his thoughts on Ken's post. In his post he brings up the very interesting idea that adventurers might treat the megadungeon the way early explorers treated uncharted lands in a world with little magical healing. The adventurers would set up base camps as they went and return to them to heal. This could be combined with something like the entourage system to alleviate the 15-minute dungeon day concerns. With a robust downtime system for the camps, it could create even more interesting opportunities and a more strategic level of resource management.
I think a game like the one described in the original post could work. In fact, I am a bit excited to try it myself.