Wow introduced new ways to fight bosses. Don't stand in this, dodge this, dance around the boss arena. When WoW came out, this was a new concept and everyone who played had to learn. Eventually, people got used to the mechanics and they became the new way to fight bosses. Now, these mechanics have been around for almost 10 years now, a lot of us are used to them, and this game employs a lot of them. It doesn't add anything super new, the fights just have little room for error.

Saying T1/T2 is easier than MC is a skewed statement due to the fact that we (as gamers) have had 10 years to get used to the mechanics most boss fights use. It's not a sense of the game being easier, but us being smarter. We know to dodge shit, we know how to figure out mechanics, and we also know how to youtube a lot of fights so we can watch successful people do them. We've adapted so the comparison of MC/Onyxia to Coil is pretty skewed.