I'd say all of the above, to an extent. The first time you do anything new, be it visiting a city or raiding in a video game, there is a certain significance to the occasion that can never truly be replicated. As the first big raid most classic WoW players experienced, Molten Core has had a special place in our collective hearts for a long time now. Let's take a trip down memory lane with a look at some of the unique and fun aspects of Molten Core, many of which I miss but honestly would not want to have to deal with again.
Perish in Flames
There's some sort of metaphorical significance in how Molten Core even had a unique mode of entry compared to the raids that followed it. Lothos Riftwalker is the NPC who gives you the MC attunement quest. These days, you can talk to him and he'll teleport you to the raid, but originally he didn't do that. Once attuned, you had to jump out of the window next to him and if you were lucky, you would hit the invisible portal underneath and the game would take you to the raid instance. If you weren't lucky, the game would simply drop you in the lava below, resulting in an unpleasant swim and/or firey death. Despite the modern talk-to-teleport option, the invisible portal still exists, so go throw yourself out Lothos' window next time you're in Blackrock Mountain and see where it takes you. If you're feeling lucky.
Caverns and Tunnels
I love big, scary-feeling dungeons. There are a lot of raids that fit this bill, especially older raids. Pretty much all of the Blackrock instances fall into this category; Blackrock Depths and Blackrock Spire (both Lower and Upper) are still some of my favorite dungeons for atmosphere. Molten Core is in the same instance family, though it does have quite a different aesthetic. It's a dungeon that, to this day, even since the addition of instance maps, still occasionally gets me turned around. Beyond the immediate entryway just after the instance portal, the dungeon branches into multiple pathways that filled with nooks and crannies, connected by ramps and caverns that lead into and over each other. You can go up the ramp and dive-bomb Golemagg from above, if you like! Don't forget to take a close look into the window behind him; you can see Ragnaros sleeping.
It's true that 40 man raiding does have a grand, epic feel to it that is difficult to reproduce in a smaller group. Even after the release of The Burning Crusade expansion, a lot of Molten Core bosses were definitely not facerolls, if only for the simple reason that they originally required nearly twice the players to take down as TBC raids. The big outdoor bosses of Pandaria such as Sha of Anger do have some of that huge group grandeur to them, but as one-offs they really don't recapture the feel of running through some enormous dungeon with 39 other people. Ragnaros has easily one of the most iconic bits of boss dialogue in World of Warcraft, "TOO SOON EXECUTUS..." which has of course become its own point of reference in WoW community. Nonetheless, ask some old timey raiders if you can, and many will tell you that the first time they faced Ragnaros, that scene sent a chill down their spines.
Raid bosses drop raid gear, and Molten Core gave us the first of the tier sets, a bit of a landmark in retrospect. A lot has changed since then. There were no multi-class tier tokens, for example, so you just had to hold your breath and hope that the piece you needed would drop. Oh, and are you a Horde raid? Is that the Lawbringer Belt on that mob's corpse? Oh well, too bad! Honestly, I doubt many people really miss that.
So, why the nostalgia fest? Well, as we move into tier 15, it seems like an opportune moment to look back at tier 1 and see how far we've come. Things were really different back then, and yes, there are some aspects of that earlier time that a lot of us miss. That being said, change is inevitable, and I'm glad that WoW is not a stagnant game. It's growing and evolving, and so are we, its players.
Now, if you'll excuse me, after I finish my list of dailies, I'm going to go jump out of Lothos' window. For old times' sake.