I think y'all are on to some important issues here. If your group is willing, my inclination would be to approach them by houseruling
4E to bring it more in line with the old-school rules that gave rise to the megadungeon style of play. In particular:
- Experience points are awarded primarily for finding treasure and surviving long enough to bring it out of the dungeon.
- Wandering monsters are a constant hazard with a fixed and more-or-less known to the players rate of occurence, so that the decision to search every cranny is balanced against the risk of an unplanned encounter.
- Magic items are mysterious (there's no easy recourse to the
identify spell) and as likely to be a bane as a boon (e.g. cursed items, intelligent swords with conflicting agendas, etc.).
The reason that these are important to the megadungeon is that the dungeon is supposed to be an intrinsically dangerous environment, at best a constant impersonal hazard and at worst an enemy in itself. Mapping is essential because nothing is worse than losing your way back to the surface. Your initial focus isn't clearing out every room for experience like in a CRPG (although that may happen over time); the megadungeon should be about going in, exploring, and making strategic choices (do we open the door with the spooky noises, or push on further; at what point will the depletion of our resources, from HP to torches, force us to turn back given that we're likely to face a number of wandering monsters just trying to return to safety) balanced against the certain knowledge that the megadungeon is full of things that will eat you for lunch if you're not both careful and lucky.
If killing monsters is the main source of experience as per the
4E RAW, there's going to be a strong incentive for the players to treat each encounter as the next step towards leveling up - not a potentially much-more-lethal-than-expected hazard that's better negotiated using brains rather than brawn. I'd eliminate or sharply reduce the XP award from monsters, replacing it with XP from treasure awarded (or quests if you want to be a little less old-school).
This ties into the wandering monster issue - if the PCs are noisily bashing down doors, you want the resultant increased risk of a wandering monster to be a punishment, not a gift of XP. This is especially true because the random factor might make the gift a trivial gimme - fire beetles! - or a Trojan horse - trolls! - so again it's important for the players not to have a system-reinforced expectation that monsters are there to be killed. The other
4E problem with wandering monsters is that combat takes so much longer than in the old-school. You want the fire beetle encounter to be a punishment for foolhardy play in that it dings the PCs by a few hit points, not in that it forces the players to wade through an hour of dull (because ultimately unchallenging) melee. Mike Mearls has a blog post about using skill challenges to handle wandering monsters - I think at his Keep on the Gaming Lands blog, although there's also a related idea in his discussion of converting the G series at the Wizards site.
Finally, magic items are a problem, as you've noted, because one virtue of the megadungeon is that it's entirely up to the players which direction to head, making it hard for the DM to parcel out the items
4E expects. And that expectation is counter to the old-school feel; a cursed item should be like "well, I invaded someone's house and caught athlete's foot from the shoes I stole, I guess that's what I deserve" instead of "these shoes I got for my birthday have a fungus?!?" What I'd do is to abstract out magic item enhancement bonuses, similar to how it's done for NPCs. When you hit second level, choose one item (armor, weapon, implement, etc) to receive a +1 enchantment bonus, which is conceptualized as just another benefit of increased experience; it's that you're better with your sword, not that your sword started to glow. At third through fifth levels, choose another item; at sixth level, one item gets bumped to +2; and so on. The wondrous items you find in the dungeon will contribute the other aspects of
4E items (e.g. item powers), and since players are reassured that the PCs will keep up with the expectations built into the system, they ought to be a lot more open to items that have unknowable / undesirable / unreliable "special" effects.
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