Five-man tuning at the start of Cataclysm was perfect; running those dungeons near launch with a group at the minimum ilvl requirements was some of the most fun I've ever had with WoW. The lack of normal modes to choose from was a problem for PUGs, but there's no reason a group of five random unskilled players should be able to one-shot the most challenging five-man content in the game. Players were just spoiled by Wrath, and had learned that "heroic" should mean "trivial." Pandaria solved the problem by making heroics the new normals, and by replacing true heroics with challenge modes.
I'm not currently subscribed, but that has nothing to do with the current quality of the game. It's just plain old; there's only so many new ideas Blizzard can come up with to keep it feeling fresh, and they're running out.
The important distinction is that you played these dungeons with guild mates and most people played them with 4 random strangers. The tuning really was perfect for premade groups, but it's clear that the majority of the playerbase is no longer interested in putting any more effort into forming a group than checking a box and waiting for the game to teleport them.
I would agree that cata launch 5-mans were some of the most fun I have ever had in the game, but they were a bad fit for most of the people that play WoW.