I agree with the OP. I've played MMOs for over a decade now, started in Everquest and played from WoW launch until I finally grew tired of it after killing HM Deathwing in Cataclysm. Everquest was some of the most fun I've had whilst raiding in an MMO. 72 man raids, extremely difficult and challenging encounters on an epic scale. I've done 40 man raids in WoW, I cleared the original Naxxramas in 40 man and I was in a top 50 in the world guild through all of TBC and most of WotLK.
I loved the WoW model of doing PvE, especially in TBC and to a lesser extent WotLK. It was HARD, very hard. You had to be dedicated and highly skilled to beat this stuff, even then it was a process of gradual refinements of strategy and play of 1% here and 1% there that finally gave you the edge to beat the fight, often after weeks or even months of attempts. My guild worked for weeks to beat Kil'Jaeden and M'uru, 100+ attempts on both of them. We loved every second of it too. We were fine when the encounters were subsequently nerfed too, I accept that not everyone has the ability, commitment or just patience to devote that much effort to beating an operations encounter.
I was hoping to see this level of challenge for TFB, we were disappointed. Everything was a 1 shot or a 2 shot, with only TFB itself giving us any real resistance. The only reason it took a 2nd night of attempts was due to one of our tanks having flaky net and us calling it due to that.
I was hoping EC NM would eclipse the challenge for TFB, it did...sort of. We took maybe 4 or 5 attempts to down ZnT, tanks not many more and we 1 shot Vorgath (well done tuning that one EAware). Again, only Kephess gave any real resistance, but we still downed him in the first week.
Granted, we're a strong group of players and we did clear the place 2nd in the world in 8m, but ... we really would love to sink our teeth into a meatier challenge. Even if you have to tune it down in the weeks after to make it more accessible, not everyone is going to be challenged by this luke warm content you're putting out there right now.
I strongly suggest your guild to try 16 man if you are in love with true challenge.