C'Thun was literally impossible for any guild to kill when he was first implemented. Phase 1 was doable, but phase 2 was just downright brutal.
Giant Claws had >90k hp, Giant Eyes had >40k hp, the Little Eyes had roughly 2.3k hp. Every 30 seconds, 8 Little Eyes would spawn around C'Thun, and mindflay the raid (if left alive long enough they would kill the healers). On another timer, every 40 seconds, C'thun would spawn a Giant Claw and Giant Eye. The Giant Claws would do about 1.5k-2k (which was a lot pre-BC) on t2 geared tanks and could use Thrash and kill a bunch of DPS or Healers at once. The Giant Eyes had a Green Beam effect that could chain to multiple targets and could kill your entire raid essentially.
When entering Phase 2, C'Thun has 100% HP and now has a protective shield that basically nullifies any incoming attacks to do basically no damage. To damage/kill C'Thun, he would "eat" people every 10 seconds and place them in him stomach. In his stomach are two Flash Tentacles which have 24k hp. Your raid needs to kill both Flash Tentacles to weaken C'Thun. After the nerf (not sure of how many before) it would take 2-3 weakenings to kill C'Thun.
Given the information above, it required guilds in T2/AQ40 gear (Naxx wasn't out yet, even though the gear wouldn't have been significant enough to kill him still) to put out a whopping 230k damage to keep up with the various spawns. Now, this was possible for guilds back then, but only on single targets. When you have 8, 12, 17, 22, or more adds alive (the spawns had no cap limit, you could technically have a full room of tentacles) its very hard to push out 230k damage for your always having to switch.
Patch 1.10.1 (hotfix)
We have also hotfixed the following changes to the C’thun encounter in Ahn’Qiraj:
#Lowered the hit points of both Giant Claw Tentacles and Giant Eye Tentacles.
#Decreased the damage caused by Ground Rupture, and made the effect resistable.
#Significantly decreased the melee damage done by Giant Eye Tentacles.
In the 1.10.1 patch, we also changed the way that C’thun identifies who is in his stomach. The faction changing debuff that was previously applied will no longer appear, but C’thun’s tentacles should not spawn in the stomach, nor should a player in the stomach be the target of C’thun’s Eye Beam.
The combination of these changes should give players more time to damage C'thun instead of dealing with tentacles, and see fewer "unavoidable, random" deaths occur due to Ground Rupture effects.
Patch 1.10.0
Temple of Ahn'Qiraj
# Removed the stacking acid from the Flesh Tentacles in the C'thun encounter.
# Digestive Acid now increases its effect over time during the C'thun encounter.
# Fixed a bug that could allow for the Digestive Acid debuff to be removed.
# Lowered the amount of knockback dealt by Giant Tentacles.
# Tentacles should no longer spawn on players who have just been knocked back by a previous tentacle.