|Go big, or go home.|
Pretty much the cornerstone of our hobby, man.
The first RPGs sprung from wargaming sessions which were conducted on a much smaller scale, using indoor environments to allow for a much more claustrophobic experience, similar to other fantasy literature where caves and lairs were far more commonplace (the archetypal dungeon is still Lord of the Rings' Khazad-Dum, the Mines of Moria, the ancient Dwarfhold where great evil lurks...).
By now, though, the term "dungeon" has been expanded to mean any restricted area of exploration - you can have dungeons underground, or through the streets of ruined cities, or a particularly dense chunk of forest, or an underwater kelp forest, or the lair of some great Evil, or a particular path through the Underworld, or an old crypt/catacomb, or a warehouse filled with storage containers, or a derelict space ship, or...
You get the idea. A dungeon can find its way into nearly any game (my Rogue Trader game has had a few dungeon crawls, mainly on abandoned Space Hulks), as it provides a good encounter - a restricted area, filled with traps, combat, puzzles, and mysteries to be solved. They make for a plot and hook all in one - PCs are the kinds of people who know that dungeon = treasure, and cool stuff. If there's a dungeon, they're going into it. No need to lure them there with hints to greater horrors, over-arcing plots, etc. - it's like a mountain, it's been put there to be climbed. Or delved, as the case may be.
For a while, I moved away from dungeons, decrying them as "immature" and "a poor man's way out" - if you couldn't make a good, compelling story without resorting to a dungeon, you weren't doing it right.
Then, I found out - I was just rotten at running dungeons. And, I'd been running them straight from pre-written adventures, and man, that's just wrong.
I had my little realisation as to how much fun you could have with a dungeon way back when I was running a one-on-one game for my then-girlfriend (the third post I ever made on this here blog!), when I designed a simple dungeon for my introductory adventure. I sat and planned all the wee bits out, making a simple dungeon - then I added a few traps. Then a few more interesting details, and rolled on some tables, came up with a full dungeon ecosystem... I got sucked in. And I had loads of fun - and she loved it. Something so simple, that can be expanded upon to make something so complicated and devious...
I now love dungeons. I try and run a few pick-up dungeon-crawl games whenever I can, simply because they're so much fun. Even just for a one-off session, regardless of the system you're using, they're a great way to spend a few hours with friends. Or strangers. Or anyone, really.