Occasionally it's also been proposed to re-randomize all the gear. We didn't have best in slot lists back then. People weren't starting out as a fresh level 1 character with a complete list of which dungeons to run at 60 in order to gear as quickly and efficiently as possible. Now, all that data is available. If gear were re-randomized then all of those lists would become useless and thing would be close to how they were. Which is a more important part of the "vanilla experience?" the fact that item X from boss Y has exactly Z stats on it, or the voyage of discovery? Personally I think that voyage is way more important. Do you remember how many points of which stat on the ring or whatever that Darkmaster Gandling drops? I don't. Would it kill your experience if the BiS for every class were in a different dungeon so people had to actually play to figure it out rather than simply consult a spreadsheet? I don't think it would. For that matter, it probably wouldn't be a bad idea to handle it intelligently so that the better gear is in later dungeons. I remember there because some weird cases where level 40ish gear was better than anything that dropped in level 60 dungeons.
That's not to say that everyone has an unrealistic vision of what vanilla WoW was like. There were already plenty of people on the WoW Classic forums pointing out that perceived bugs are just recreations of the original game. And as noted before, a flaw to one player is a key component of the original positive experience to another. WoW Classic will surely please plenty of purists in spades. But this goes to show that it won't be for everyone who has fond memories of the game circa 2006.
Within the context of Classic, the method most frequently proposed is to re-tune bosses to make them harder. Rather than copying the numbers, re-create the relative difficulty. Kind of like, when you first start working out, you use small weights. When you get stronger, you lift bigger weights. The WoW community is more knowledgeable, more experienced, has better hardware, etc. We're "stronger." So give us tougher bosses. There are a lot of forms that could take. Bosses were constantly re-tuned during vanilla anyway, so if Ragnaros does 10% more damage or something, is that really a problem? I don't think it is. What about resistance gear? back in the day, collecting resist gear was a thing people did. But the numbers and strategies are so well known today that only the tank really needs it anymore. Bosses could be re-tuned with more resistible elemental damage so that's important for entire raids to gear appropriately. That would be entirely keeping with the spirit of vanilla even if the actually numbers sitting in a database somewhere were different than they were ~13 years ago. If you want to go a little farther, a few people have proposed adding new abilities to old bosses, adjusting their timing, etc.
Some quests in my guide are marked as "SKIP" and colored in red. These quests are simply either too hard to solo or not worth the XP/time and are skipped. My guide will only list SKIPPED quests if the quest is a direct follow up after completing a quest, not one that you have to click the NPC again to get it. If you hover over the skipped quests, it will give info on why it is skipped in the guide (unless that info is already listed directly in the guide text).
So committed, in fact, that modern WoW players are trying the beta and reporting what seem like bugs today but what were actually intended functionality 13 years ago. This became such a common occurrence that Blizzard publicly posted a list of known non-issues called the "WoW Classic 'Not a Bug' List." For example, hitboxes for the Tauren player race are much larger than those of other races. In a modern game, this would be seen as a serious balance issue (see: Apex Legends). But it's what vanilla WoW was like, so it has been faithfully reproduced.
All of this is keeping with the spirit of vanilla, which I think is more important than that a specific number be 11 instead of 12. And it would help to "resize the swing" by making the game harder to the more experienced community, and forcing people to re-discover the game rather than simply looking up whatever they want to know on a spreadsheet somewhere.