My vote is for lame.
Of course, I think that Orcus and Grazz't (or however you spell that) are lame too. The traditional demon princes of D&D are just a big collection of lameness. This is a result of the fact that traditional D&D depictions of the Abyss itself are equally lame.
Look at Demogorgan's domain, Abysm. It's a jungle surrounded by a briny sea, that is populated by mortal races like lizardmen, yuan-ti, and troglodites, and the native threats include dinosaurs, dire apes, and demonic monkeys. What the heck are these tame threats doing in the middle of the Abyss? The Abyss is supposed to be a swirling mass of absolute chaos and destruction that is slowly eating up the entire universe. It is the most dangerous region in the entire cosmology, where only the most powerful and epic of heroes dare tread. Yet, traditional descriptions of locals in the Abyss are surprisingly hospitable to lowly Heroic and Paragon tier heroes. I just don't get it.
It made sense in Planescape, where planar adventures were something you can build an entire 1-20 campaign around. In 4E though, locations like Abysm just don't seem to belong, and old demons like Demogorgon just don't seem to fit.
The thing is, I can easily imagine Demogorgon being a very powerful and malevolent Primal Spirit or Feylord, with Abysm being his domain in the Feywild. Thanatos actually makes more sense as a particularly dark and dangerous realm of the Shadowfell, with Orcus as its Dark Lord. But, I just don't thunk those two work as demons.
When I think of a demon, what comes to mind is something like Ragu O Ragula, from the Wild ARMs series of videogames. This is a pretty good video of it in action:
[ame="http://www.youtube.com/watch?v=krAZelLnHiM"]Ragu O Ragula[/ame]
