On its own, it's a good raid. Solid environment and mechanics. Nothing wrong there. Well, there was one thing wrong, but I'll get to that.
However, there were significant issues that made 6.2 an unenjoyable and regretful patch when it came to raiding. Most of them revolve around the legendary ring.
The legendary ring was an unmitigated disaster. The obvious immediate issue is the trivialising of the raid encounters, but ultimately that's not so bad especially for guilds who were stuck on bosses in there in a raid that was out for a year. What really fucked it up was how it influenced class balance. The legendary ring put immense value on the DPS you could output in that 2-minute ring CD window, since not only did you get something like a 45% damage boost at maximum item level but it also duplicated that damage in the explosion at the end of the duration. That meant that specs that could output the most damage in that window were at a huge advantage while specs that couldn't were at disadvantage.
This led to some of the worst tuning imbalance we've seen since Vanilla/BC. You were looking at a 40% difference between the top and bottom specs in the game POST-HOTFIX. Many specs were made flat-out unviable. For example, you would be insane to play a Mage and not be Arcane (except for heavy AoE), or a Rogue and not be Subtlety, or, most importantly to me, a Hunter and not be Marksmanship.
On the Hunter example: Hunters, like many classes, had a rough launch going into WoD with MM being the only viable Hunter spec on all fights, but that was fixed early on and in Highmaul and Blackrock Foundry you could conceivably play any of the three specs you wanted, and people did. I had a blast switching between the three basically whenever I felt like it. Survival was the best in Highmaul, BM was the best in BRF, but you weren't SOL if you didn't play those. Then HFC comes out. Because of the legendary ring (and the super OP 4-set bonus and priority add burst, but it was primarily the ring) MM was the ONLY choice on pretty much every fight as it had unparalleled 2-minute burst from Rapid Fire and how that worked. BM was usable on a couple if you were doing a heavy AoE strat but MM was still generally better because a) BM's cooldown phases didn't necessarily synchronise with that 2-minute ring window and b) BM didn't have as much of a damage hike during that window as MM did. Then there's Survival. Survival didn't have a single baseline cooldown, period. It was entirely sustained damage. Coupled with the ridiculous Serpent Sting nerf, it's no surprise it ended up being bottom of the barrel. Before hotfixes you were probably better off bringing a 3rd tank to DPS instead of a Survival Hunter.
So we were all stuck as MM for the entire tier. What's worse is that the tier lasted a full year. What's even worse is that at the end of that tier we got Legion which basically redid every Hunter spec... and not in a good way. Turns out HFC was Survival's last raid ever in its ranged form. It went melee in 7.0. That's another discussion, but as a long-time Survival player it would have been really fucking nice to have a proper send-off for the spec. "Hey, you know that spec you love and have been playing for 8 years? It won't exist in 12 months, so we'll let you enjoy it in HFC while you can". NOPE. I couldn't play it in HFC because my raid depended on me being MM. Seriously. There were bosses that I doubt we could have killed at all if I were SV instead of MM. Sounds ridiculous, but look at Gorefiend. If I wasn't there that day, the raid skipped that boss. Seriously. It just wasn't an option, and that was the "last hurrah" they gave to the spec. Broken and unviable, with everyone forced into MM for the whole tier.
TL;DR: The Legendary Ring was a major factor in forcing people into single specs of their class for a year regardless of whether or not they liked that spec.
P.S. Remember how I said there were some things wrong with the fight design? That was the priority add mechanic being on every single damn fight in there. Seriously. Voidfiends on Xhul'Horac, ghosts on Gorefiend, bombs on Iron Reaver, dreadlords on Mannoroth. This is another factor that made certain specs overwhelmingly better than others like, say, MM which had Careful Aim + Aimed Shot for immense DPS right on add spawn v.s. SV which was entirely sustained damage.
EDIT: Here is the simulationcraft for that tier. Again, post-hotfix.
http://www.simulationcraft.org/reports/Raid_T18M.html
Holy hell. Switch from MM to SV to lose 20% DPS! And this was for pure single-target Patchwerk, i.e. SV's best-case scenario. It was even worse in real fights with priority adds.