Here's a quick dungeon design checklist for if you're planning a session, making a map, or tossing something together. The essentials.

Story

Story-wise, a good dungeon...

• Has a name
• Has a good why/origin for existing
  • Has a history
• Ties to a good why/reason for being there
  • Tie it in with the overall campaign, adventure, story, or characters
• Doesn't make it just us vs them
  • Give inhabitants complex motives
  • Think about the ecology, economy, and cause & effect
• Engages the senses
  • What do players smell?
  • What do players hear?
  • What do players feel?
  • What do players see?
  • What do players sense?
  • What do players find that's interesting other than treasure?
• Considers what happens after time
  • If they stay too long, what happens?
• Challenges your own and the player's assumptions
  • Not everything has to make sense
  • Not everything has to be realistic

Play

Play-wise, a good dungeon...

• Has a focus on exploration, social interaction, and/or combat
• Balances between modes of play
• Engages character and player strengths and weaknesses
  • Make sure the dungeon has something for each player to do
• Balances risk vs reward

Mapping

Mapping-wise, a good dungeon...

• Has rooms that are labeled, not just numbered
• Refers back to a mini-map if it takes multiple pages to describe
• Supports different tactical styles of play, where possible
  • Near vs far
  • Elevation
  • Choke points, pincer attacks, etc
• Provides multiple entrances/exits, especially for larger dungeons
• Avoids the need for a legend to make sense of any symbols
  • But definitely explains any symbols if it needs to
• Provides secrets to uncover, even ones no one might find
• Includes what's important, and omits what's not, keeping things quick and easy to read