The Obsidian Sanctum was a small raid with basically one boss (Sartharion) which was doable very early on in the expansion Wrath of the Lich King. Only a tiny bit of coordination was needed to bring down the boss. Because of its size and difficulty level it was very PUG-friendly. Heroic dungeon gear and a few pieces from Sartharion and you were set for the 'real' challenge: Naxxramas.
(The Eye of Eternity was another small raid which (if tuned down) would have made a great addition to The Obsidian Sanctum as one out of two 'expansion-entrance-raids'.)
Imo this was the perfect solution to cater to every kind of player. Some would skip the entrance-raids and start with Naxxramas right away (the hardcore), others would do Sartharion for a few weeks until their ilvl would make Naxxramas easier (the casual raider) and some would do Sartharion and heroic dungeons until the next patch and then with the gear of the 'new-patch-heroic-dungeons' they could join Naxxramas raids (maybe with mounts dropping there as an incentive for all players to visit the raid again).
Like this everyone would experience the story of the expansion (maybe a little delayed) AND you would still get that feeling we all got when we first beat a 'real' raid boss (e.g. a casual player is progressing and becomes a casual raider).
Wotlk was the expansion where I went through exactly this transition. I was a complete noob doing heroic dungeons and Sartharion every week on my blood elf rogue and then one evening I joined a group for Naxxramas (I was shivering...ecstatic). Most of the 10 player raid I was in was inexperienced and it took us quite a while but a few hours later Patchwerk was sparkling in front of us and we were cheering in the call. It was amazing. That was the moment I started to invest more time into raiding and in the end of the expansion I was raiding ICC on heroic. This is simply what the raid aspect of this game is about: wonderful moments that you progressed towards.
I'm not saying LFR is bad but...if I had seen patchwerk sparkling for weeks in LFR before...it just wouldn't have been the same.
my post is about what the way of progression should look like for casual players/raiders.
I don't care how easy T7 was for the hardcore raiders. You have mythic now. Blizzard has got you covered and it will never change. For a while now the discussions have always been about the way Blizzard caters to casual players/raiders.