I think people are putting difficulty completely down to mechanics. I completely agree that the mechanics of all most every boss fight over the past 4-5 expansions is "Mechanically" more difficult than the hardest bosses in Classic, however... One of the hardest things to overcome in Vanilla is simply damage values and class limitations. Healers ooming and tanks getting smashed extremely hard is the biggest problem all the way through Vanilla, especially early on. Bosses in Mythic might take a bigger percentage of your health away, but healers are more equipped to deal with that now. Every single healers has like 2-3 insta-heals and the amount of HOTS flying around means raid healing isn't a problem anymore. If you're on alliance side in Classic pretty much every single raid heal has to be manually done, you can't rely on chain heal to do the extra healer for you.
Another thing to consider: Most mechanics nowadays aren't random. What I'm implying here is that you can barely blame RNG for wipes anymore. Almost every wipe is down to player error. In vanilla you can get randomly killed by the very first boss (Onyxia) simply because of an unlucky fear/cleave and there's also the chance of being feared into whelps. There are a lot of situations where things like this can happen in classic and I think it has to be taken into account. It's not difficult by definition, but it's another annoying thing that will affect a guild progress.