It's all just brick walls and mold everywhere! Oh Noooo!
I feel like so many DM's need a Queer Eye for the Strait Guy on their dungeon designs.
Basements, tunnels, and larger cavern structures are fun but what we need is more tombs filled with pictographs, natural formations jutting through structures, actively used old buildings with local friendly or no natives to deal with. Or just take a moment to think what was the roll of the Dungeon before it became what it is today.
Lets talk about layout:
I see a lot of dungeons have a hap hazard design. Like the person just HAD to fill in the grid map. Which is fine, but consider living in that place, much less using it. Who the heck has maze like hallways unless the dungeon was specifically designed as a labyrinth to fool invaders. And if that is the case did the designer ever put in road marker clues just so THEY wouldn't get lost? What if the dungeon was a specific use like an old prison? Wouldn't large open work yards and smaller cells make more sense? What about guard rooms and facilities for them? And if it is a castles treasure room, where is the layers or protection, hallways suck at this.
When thinking up a dungeon's lay out look at the entrance, and local buildings. Think about what had to be pulled in and out of there and what the designers would do to get the most out of it. it can be dank...sure...but why? Why not have someone clean it here and there. Have Orc guards keeping an eye on well lighted areas. Have peek holes, and hidden supply closets, etc. What if it was just a mass elaborate trap?! You have to consider these things.
Take a minute or two to ask yourself the following -
What was this dungeon used for?
Is it maintained?
What structures would be useful for it's previous owners?
Now lets talk about location:
I guess it's easy to build a dungeon under a castle or into a mountain. But what happens if you are in a desert? Or if you have a massive island nearby? Or if you want to hide it in a city?!
Location, Location, Location!
The classic dungeon tends to be located in crypts, keeps, mountains, etc. Look at a lot of the art of the dungeon itself and you wonder if you are trying to traverse the fantasy version of the Maginot Line (note to self...) with all the weird interconnected tunnels and rooms. But environmentally you have other options. Look at Pathfinder's ruins from the Runelords. One dungeon is under a massive bridge complex, another is a giant face carved into a mountain like something out of Zardoz, and there is my all time favorite scratched space ship. (Which D&D did first, yes and it was a damn good module.)
But also look at JRPG games to see Dungeons that run from large crystal caverns, weird old techno ruins, the living guts of a massive boss, floating sky castles, and other fun and dangerous variations. The better dungeons I've played in really cranked up the environmental hazards around and inside the dungeon itself. Half Flooded? Suspended over lava! Go for it.
Finally it's more than just window dressing:
Good Dungeons live and breath based on the elements you use to describe them. Lets look at Earthdawn a moment. The idea behind Earthdawn was to cook up a setting that took all the tropes of your classic D&D setting and give them a semi-logical reason behind them. The Horrors taking on the world, the massive underground cities, the strange casting mechanics, etc. But what sells the setting is the Kaers themselves, the massive underground city shelters that civilization used to avoid the horrors. All the adventures into one detail the actual layout of a real town full of people. There is something very FALLOUT like about the descriptions, magical support (life support) rooms, artificial light gardens, tunnel warrens of homes, community centers, and then they invert it into horror when one of the demon-like Horrors (big H here) breaks in, undeads everyone and runs the Kaer like their own puppet theater. And you get to enter into this. Right there, just the creepy effect of "people actually use to LIVE here" ups the tension, make the classic dungeon 100% nastier, and keeps the players on edge.
Other good descriptions include actual real world ruins. Did you know Egyptian tombs actually have traces of paint on the way over the hieroglyphics? Or imagine running across orc cave paintings like the early human works found in caves? Mix in things like Mayan Sacrifice pools, maybe a dash of ball courts, and a pyramid INSIDE another pyramid (common practice!), and things get a LOT more interesting.
The classic 10x10 room has it's place in D&D history and it's always fun to run into a classic Tomb of Horrors or Undermountain, but they do get boring. Keep trying to spice up your underworlds, your castles by keeping the suggestions in mind. It will spice things up a little and improve players expectations.
And make things Swanky!