There's nothing wrong with the fluff in the Yeenoghu article. Witness discussion of coolness. But the article also shows what I think is wrong with 4e.
First, what is up with all of his immobilizing attacks? When he flails someone and they can't move for a round, what does that look like? Is it supernatural? Are they off-balance? Is it an abstraction of his speed and ferocity, relative to the PCs?
Second, he and his aspect are different "classes." That wasn't true in 3e; aspects were medium HD outsiders with relatively simple powers. But they had the BAB, saves, etc, and of the more powerful being. In 4e, Yeenoghu is a Solo creature, but his aspect is a Skirmisher, one that is implied to run with a pack of gnoll allies. There's no basic problem with the idea of different Yeenoghus having a different feel, heck, you could write different versions of the demon lord himself for different encounters, but there is, for me, something of a continuity problem there. There is no "big" Yeenoghu and "small" Yeenoghu, instead, you have a Solo creature and a skirmisher. They are both different in level and different in kind, which means it's hard to guess when and how to use the different versions as encounters. In 3e, you could glance at CR, hit dice, movement modes, and main powers, and guess what to do with them. In 4e, you have about the same, but XP value is much more fluid and the different versions have less in common mechanically.
It raises weird questions, like, would you come up with different versions of a juvenile red dragon that served, respectively, as a boss monster, a mount, or part of a war party?
Third, nearly everything about his attacks is a special case. So... can someone disarm him and use his flail? If so, what powers does it have? There is no separation between the monster and the gear. There is no simulation, no "NPC-like" robustness that allows us to pull apart his stats and find some underlying traits we can use to answer unexpected questions about him. How much damage does his flail do? Does it immobilize in the hands of others? Is it a Superior weapon? And so forth. 3e didn't cover everything, but it had the greater issues covered. AD&D didn't cover as much as 3e, but it tried to cover enough.