Fact is MC was never difficult. We just plain sucked a decade ago. Surgers were a nightmare IF half your raid fucked around and didn't stack. A joke if you stacked. Garr was HELL if tanks didn't pull adds away from the healers (boom, 5 dead healers). This even happened on Nostalrius if attention-spans lapsed.
Killing Ragnaros pre-sub in patch 1.1 was difficult though. Crappy talents, shitty gearing, people stacking, no widespread potion-spammage....
On Nostalrius, with proper boss-stats, and before DM, a good guild had 0-1 submerges.
Decent guilds had 1-2 submerges.
DM-patch changed this, with the addition of new and much better items to put in our BiS-lists.
Pre-BWL/ZG-patch improved caster-dps even more, with the addition of green "of Frozen Wrath, of Shadow Wrath, etc" items, which became BiS in many slots until the release of BWL.
Pre-DM a top-geared mage had in the ballpark of 150-200 +frost damage.
After these pre-BWL patches hit, he had 400-500, depending on lootluck. These patches also added most of the sexy caster-loot to MC, which didn't exist on launch, while also buffing the power of existing items.
On Nostalrius PvE, I had myself around +700 frost damage every raid because I stacked consumables, just for fun. Flask, elixirs, oil, food, etc... and this was before any ZG or BWL loot was available. A few MC epics, but mostly blues and greens.
I don't have the numbers in front of me, but let me give some examples off the top of my head about how the BiS-items changed from release until the pre-BWL patch:
Head: From +18 dmg to +39 dmg.
Neck: From Int/spirit to +35 dmg.
Shoulders: From +18 dmg to +29 dmg.
Back: From +7 dmg to +20dmg.
Chest: From +35dmg to +40dmg.
Wrist: From Int/spirit to +19 dmg.
Hands: From +18 dmg to +27dmg.
Belt: From 1% hit to +18 dmg.
Weapon (2h or 1h+oh): From 1% crit / +11dmg to 1% crit / +75dmg.
These are estimates, but quite representative of how early Vanilla itemization was for most casters.
TL;DR
The bosses weren't harder in early vanilla. The talents, but mostly the gear, just plain sucked.