Vanilla dungeons had several redeeming qualities, but many think they were mostly time-consuming and confusing to navigate. Let me take you back to a different era...
You can't talk about old school dungeons without mentioning Blackrock Depths. Crocolisk Brain. The dark iron city was a piece of art; players would visit the underground maze many times before they reached max level, and again later on. Layered exploration was the name of the game, as you would unlock a bit more content everytime you revisited the instance.
Once I was thinking of making a list of all the things its used for, shit you can do and use it for in all kind of shenaningans, and its mindboggling really, how many stuff there is. and the more you play vanilla the more you know all the application, from various farms and applications for different classes, various routes, stealth runs, arena runs, arena random bosses spawn is really interesting and unique on its own, lava runs, ace of elems farm, regs for different things, dark iron ore runs, blacksmithing forge, enchant regs, recipes, bag, bis items, t0.5 chain and so on. a lot more. its literally INSANE.
...and that's how it was back then. Strange, isn't it?