In the last few days our guild ran two instances, Shadow Fang Keep and Gnomeregan, and the two experiences nicely mirror the differences between WoW Classic and Retail IMO.
For SFK, our group was a bit higher in levels than is recommended, and as SFK is intended to be a horde-side dungeon, we didn’t have any quests beside a Paladin on his class quest. The result was that we facerolled the instance without issue, and the experience was mostly focused on seeing what the bosses dropped for loot. The whole thing was over quickly, and I can’t recall a single boss mechanic or area of the dungeon that sticks out (which is a real shame because on its own, SFK is an awesome dungeon).
For Gnomeregan, the situation was basically the opposite. As a group we were below-level, which meant every pull was its own challenge, and we wiped more than a few times. It also led to a slower pace of progress, giving us more time to appreciate the environment and chat in Discord. We all had the dungeon quests, and that means people collecting what they needed, and hoping we got enough of everything for everyone (we didn’t).
Even on the bosses that didn’t have complex mechanics, we were still cautious and, you know, paused before a boss to make sure everyone was ready. The loot was nice, but making progress felt like the real reward.
Progress eventually stopped towards the end, in a tunnel with higher-level dark dwarves. Anytime we had a bad pull here, the result was a wipe, and its an area that getting the pull correct isn’t automatic. Lots of patrols, lots of odd agro, and all in a very tight space. We also had a WoW Classic moment where our Huntard wiped us when his pet decided to agro half the entire dungeon. We called it a night having wiped on the final group before the final boss, as the earlier areas began to respawn.
SFK was easy, boring, loot focused, and the mechanics or player skill didn’t matter. It was Retail. Gnome was harder, slower, how we played mattered, and ultimately we ‘failed’, needing to come back and try again. That’s Classic.
I don’t know how Blizzard or other devs can push people to run dungeons closer to the Gnomer experience vs the SFK one, but they should. I know there are players who think they want the faceroll easy experience, but they don’t.
Hardcore Casual 
That is kind of the way classic/vanilla dungeons were. They were all over the map for quality and mechanics. The five person dungeon content did shape up with TBC and WotLK. But once dungeon finder came in and it was decided that dungeons had to be 30 minutes or less… well, even that didn’t totally kill dungeons, because heroic was often an option, but it put an end to anything like Gnomer or Sunken Temple, the long sprawling crawls.
Counter-point: Wildstar.
Also, who you run with matters. Completely different experience with a group of guildies on Discord versus randoms from Trade chat. Very few people desperate enough to do that these days (in hard dungeons) without some extreme vetting/Gearscore metrics.
Our SFK and Gnomer groups were the exact same, in this scenario. But, I do agree, pug vs guild group can make a HUGE difference.
I wonder how much of the perceived difference between Classic and modern MMORPGs, particularly WoW Retail, comes from players simply allowing the games’ systems to dictate their play. For years, when I’ve been reading about WoW, it’s either been people talking about the raid game or how they levelled an alt in next to no time using Heirlooms, RFF and every other aid known to man.
On the other hand, I’ve read a few far more interesting accounts of people with static groups or playing solo, who level-lock their characters and play through content in chronological order. Not only are those stories more interesting to read, being much more like your Gnomeregan experience, but the players seem to be having more fun.
Not all modern MMORPGs allow you to level-lock and the increasingly annoying “horizontal” concept of every zone/area adjusting to your character’s level or vice versa almost completely removes any real options from the player. In games that stick to the old fixed level zones, though, and give you the tools to avoid outleveling them too quickly, it’s still possible to make the game conform to your standards rather than the other way around.
Also, just as a general rule, most MMORPGs are a lot more involving and exciting if you consistently attempt content a little above what the game tells you is the optimum for you. It varies from game to game but a little experimentation will find the point at which you can’t faceroll content and have to start thinking. That’s the sweet spot. In WoW Classic, solo, I find it’s mobs 2-4 levels above me and orange quests.
Some MMOs just don’t allow this though because of level scaling. I found ESO, post-scaling, immediately insanely boring, and there was no escape from that.
Also, and this is shades of grey, but if you have to go out of your way to make something challenging, that isn’t the same as simply running a dungeon in Classic once the game allows you to collect all of the related quests, which is always on the low end of the level scale.
I’m confused here. Both of these dungeons are classic ones, the only difference is that you were overleveled for one and underleveled for another. Well that and Gnomeregan is infamously difficult.
But retail WoW still has levels…doesn’t it? I haven’t played since Warlords of Draenor but I’m pretty sure.
Retail has levels, but since you can LFG dungeons in under 30 minutes, you aren’t getting the Gnome experience running a dungeon in Retail.