It was not just that dungeons were large, but they were part of the world, they seemed to have a reason to be there and felt a lot less artificial, but at the same time, they delivered an adventuring and story progression feeling. Today doing a dungeon is just like watching an episode of a tv serie: predictable, short and dumb-proof.
Maybe large dungeons were not pug friendly, but I never had issues organizing a BRD run (it could have used a less widespread level range, that's true) or a LBRS run, UBRS was initially more complicated, but still doable.
Even Scholomance which was probably the more "compact" of the level 60 ones, at least visually, had a feeling of being "real" and not artificial, with all those schools of magic, necromancers, secret passages, cellars, terraces, etc.
Stratholme was a blast, if you had a group good enough to do both sides it was even better. Dire Maul, while split in 3 wings, was still a single dungeon, you could go from wing to wing without zoning out and changing portal.
Places like Maraudon were amazingly beatiful, all my characters made the scepter to enter at the waterfall, in the same way as they all did the hammer to use in Zul'Farrak and call the beast from the pool.
I also remember those rare drops that made my day: Dragon's Call in Sunken Temple (nowadays it's a shithole in comparison to how convoluted was initially), or the various BRM trinkets: Hand of Justice above all.
Druid tanking with Green Whelp Armor anyone?
Then came raiding, the 40 players one. We all know how it went from there.
With all its defects, failures, forced roles for some classes, bugs, imbalances and so on and so forth, I still consider vanilla WoW the pinnacle of online gaming from an adventuring point of view. Gameplay was changed so many times over 10 years, but that feeling of exploration and that... "standing in awe" at the magnificience created by the designers was enough to brand my brain for life. I'm not joking when I say that I explored every single square yard of Azeroth and since I mostly played a druid I considered the seas just as interesting as the lands (most of them are empty! figure that...).
I figured out my aquatic form quest by myself after all, I found out about the Tidal Charm in... let's see who remembers that and countless sunken ships and ruins around the world.
I never got crazy for loot in my MMO life, with the sole exception of collecting FT items in EQ for my bard (I paid for them with my DKP, bitches), all I cared was exploring and adventuring and loot+levels were just the necessary evils, things to collect in order to progress the exploration. In fact even those FT items were useful to have a faster recharge on my mana and abuse Fading Memories a lot more often
Today WoW is different and I love it, but instanced content is frankly a pile of shit. Exceptions? Yes, there are always some. I like Shadowmoon Burial Ground, because it completes nicely the outer zone of Shadowmoon Valley and the Ner'Zhul storyline, it has a good atmosphere. In a way it was nice to see inner Auchindon after we saw the exploded TBC version (the instance itself is a tad boring), and Grimrail cannon boss had an interesting mechanic, but for what concerns WoD, outdoor content has been a fuckton more interesting than indoor one.
Highmaul is pathetic, most dungeons (I exclude UBRS because it's not new) are boring, too short and lack appeal from any possible point of view. Pandaria dungeons, while not amazing had a lot more soul, Cataclysm ones were even better (a couple of them are among my favourites, I don't know, I just like following Bronzebeard I guess), Lich King ones ranged from good, theme-wise to amazing (my favourites were Halls of Lightning and Halls of Stone and the ones released later, Forge of Souls, Pit of Saron and Halls of Reflections, the last one was especially good, being chased by the Lich King has no price). TBC were too linear, but they had some soul behind as well. Vanilla... ah vanilla, how I miss thee, old dungeon design.
Rehashed content: I don't dislike it as a rule of thumb, but it requires a bit of effort to be taken seriously: let's say that the remake of Deadmines was amazing compared to the remake of UBRS and end it here.