I think part of the problem is that many monsters are supposed to be scary as part of a group, rather than by themselves. Looking at a bunch of not solo monsters and seeing them as weak is ignoring that they're supposed to be part of a larger whole. A scary group of monsters will be greater than the sum of its parts.
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Some of these monsters seem pretty dangerous to me. By itself, the level 24 example lich has regeneration, an autodamage aura over a wide area, a Second Wind (with elite HP!), elite properties (AP, save bonus), can stun, and has a big damage area attack. And it's less than half of a standard encounter. Liches in general have recharge on powers that are normally per encounter.
So in the context of an encounter, a lich seems pretty dangerous. They're usually hard to take out quickly with Regen, extra defense bonuses (compared to other templates), and Second Wind (it's rare for monsters to be able to use healing surges). And that extra time to kill means that lich continues to get its good powers back while the PCs use theirs up, and the aura is inflicting more damage. Now let's add some back up - a lich works well with undead allies themetically, with his necromantic power. And mechancially - the lich has the option to dish out necrotic damage, which isn't going to hurt his undead allies. So his area attacks can be selective essentially. Stunned creatures grant CA, so lich increases the effectiveness of his allies - especially those who gain extra damage with CA or against stunned targets.
So consider the case of the lich stunning the cleric, spending his AP to drop his 6d6+ area power, and then sending some Sword Wraiths or Ghouls in to exploit the vulnerable character. Now the healer and person most likely to be doing the radiant damage needed to stop the enemy regen is taking a bunch of damage and losing his turn - so that damage won't be healed right away! Next turn, there's a decent chance (~56%) the lich will get one of his powers back to use again.
Pure, evil, genius. I Love it.