The sha buff was a response to the previous fight with a bonus phase, heroic Rag, where much like with H-Garrosh phases 1-3 were so hard that phase 4 couldn't really be hard on its own, but rather a check to see if you could make a clean transition enough times to learn the phase well enough for a kill. The sha buff made it so that they could actually get away with making phase 2 hard on its own merits rather than a "did you make it here in one piece" check.
The problem with the Sha buff was that it encouraged the weird gameplay of optimizing your comp for phase 1 for the first few pulls until you made a transition and got the buff, then switching people out to get an optimal phase 2 comp since phase 1 was easy now. Any chance of having that kind of buff again died when people started doing that.
It should be noted that the "hard phase->easy phase" fight design where the last phase is basically just a check to see if you can reliably get there with everyone alive isn't exclusive to bonus heroic phases. Razorgore is the first example of this kind of fight, and most of the more highly-regarded fights in the game have done it- Kael'thas, M'uru, Lich King, etc.
To be fair Suzu, Heroic Rag with his INTENDED mechanic would've been a much more complex phase than what we all did, and would've certainly stressed having 3 healers to the max. Plus, I would just patently disagree about M'uru (Phase 2 was absolutely the harder part of that fight). As much as the last phases of Kael and Lich King have really interesting and complex mechanics, I won't try to justify them as being harder than the preceding phases though.
Re:Garrosh, P2 was always easy, but the dps check was real enough through all phases of the fight. Not 'burn' level DPS like H Rag P3 dps hard, but Baleroc 'consistent high dps requirement' hard. The difference being of course that in a longer multiminute phase or fight you can have a high dps check that doesn't seem as punishing because it's very consistently going to be either doable for your group or not. You're not likely to be RNG teetering on the edge there searching for minute optimizations.
To me the biggest issue with Sha was that even though it was a two phase fight, phase 1 was snooze inducing, and phase 2 might as well have been split into two sub-phases: The part where it's difficult, and the part hwere it's not. I REALLY liked the mechanics of that boss, but holy shit was it boring for like 75% of the encounter. I would happily wipe after 10 minutes of a Rag pull where I was playing intensely than after 10 minutes of Sha where I was fucking sleepwalking for the first 8 minutes. Bosses that require constant attention will do a much better job of keeping your raid on point imo, and I certainly don't think it's 'intended' that the final phases are easier, it just happens to work out that way. Garrosh P4 is pretty damn complex when you're going itno it blind beyond datamining, it's easy now because it's been dissected and we know how you can trivially handle malice, iron stars, bombardments, etc. Plus DPS makes it total jokes in terms of how many times you deal with the 'bad' bombardments and malices in a kill.
M'uru's burn phase (or Phase 2) is just as hard as Phase 1. I'd say even harder because P1 is a control check and P2 is a DPS/Healing check
In all M'uru was way way harder than KJ and prety much any fight before it (I discard the 4HM since all you needed was 8 warriors with 4/8 DN)
Funny thing is, the 8 warrior 4/8 Dreadnaught thing is a super common misconception. I wasn't in Jugg at the time, but I know for instance that they killed it with like half druid tanks because there weren't that many warriors to even have in your guild, let alone with 4/8 and Nat Pagle's trinkets. The actual tank damage on that fight wasn't anything astounding, you just needed people that could rotate and taunt correctly (which druids could do since they had caster gear available at no real survivability loss).
Now I'm all nostalgic, waiting for wildstar servers to come up from patch day, There are a lot of shitty fights in wow, but the REALLY good fights will always stick out to me and remain super fun. I could go fight HLK right now like it was progression again and have a blast. Ditto two dozen other super well designed bosses of yesteryear.
Edit 5: Thinking about Sha again, P2 suffered from the same bullshit as spine, where the problem wasn't how the fight mechanics worked - they were cool enough and challenging, it was that it starts at a difficulty of 0 and slowly ramps all the way up. It needed to start at like a 5 or 6 bare minimum. IE: Sha starting at like, 4 add spawns, and having half the hp? Would've been beautiful. Spine starting on Plate 2 or something would have been much less tedious.
Found this:
http://www.twitch.tv/theyanger/c/1876942
twenty fucking minutes. And I don't even look mildly interested in what's going on until like 15 minutes in. Complete trashcan shit jesus christ.