PC vs. NPC scaling by level: PCs scale at about -1/7 levels slower than NPCs, in d20 roll categories which they are stat pumping. This hits -4 at around lvl 30, and 4 is where things *break* on d20s. It gets worse in non-pumped rolls, and there 4e doesn't give you ways to end-run like Mind Blank.
+Extra stat on d20 roll abilities start giving things like +/- 8 by the Epic tier. This can take you from 50% success rates to 0/100% success rates. Witness the Orb wizard. Or, more simply, compare the party whose warlord landed Lead the Attack (lvl 1 ability!!) against a solo to the party whose warlord failed to land it (or the party without the warlord at all). +8 to hit will at least double the party's melee output. In 3e, the assumption was that *everyone* had *multiple* ways to hit the needed damage output, and would use them until they worked. 4e doesn't give you enough similar options to make them reliable.
Ok, lets say you have an optimized PC with say a 20 in your prime attack stat. Let's say I'm playing a Fighter. At level 30, assuming I always boost STR, and take the Demigod Epic Destiny, my attack bonus is going to +3 prof. +1 fighter +10 STR +15 level +6 sword = +35. Against a suitable epic foe like an Ancient Red Dragon with an AC of 48, I hit on a 13 or better. Not too difficult, but not too easy either.
Let's say I have a warlord in the party who uses a power such as Warlord's Favor that grants me a bonus to hit such as 1+INT. Let's assume that the Warlord starts with an 18 in their INT score even though STR would be their primary and let's assume that the Warlord increased INT every level the way my example Fighter did. So the Warlord gives the Fighter a bonus to hit of +10. Pretty good. So for one round, the fighter can hit on a 3 or higher.
So, lets go worst case scenario (for the dragon). Let's assume that due to ally buffs, or imposed status conditions and whatnot, that we hit the Dragon every round. I'll assume that generally a level 30 fighter will do 5W every round with their different powers. Let's say I'm optimized with a +6 Bastard Sword and thus do 5d10 + 10 STR +6 magic weapon +9 power attack +3 Weapon Focus, let's throw in an additional +10 misc bonus for damage boosting powers and such. So on average thats about 63 damage per hit, assuming no crits.
At 1,390 HP that would take my Fighter, by himself, 22 rounds to kill the dragon. That's a pretty epic fight. But since a level 30 Ancient Red Dragon is a solo, let's assume a I have a party of 5 level 30 characters to help out. If they all generally average out to the same damage and everyone ALWAYS hits EVERY single round, that is still 4.4 rounds to kill the dragon. Assuming the dragon just stands there and gets hacked on without attacking back.
I see absolutely nothing broken about 4e's math in my example. Assuming that misses and crits balance out, I'd say 4.5 to 5 rounds for a party of 5 level PCs to defeat a challenge equal to their level is exactly right on target with how 4e's math is designed to work. Even if we assume the PCs miss half the time, the combat now takes between 8 and 10 rounds. Again 4e is designed for a standard encounter of the PC's level to take between 5 and 10 rounds. So again we are right on target.
So what am I missing? Where is the broken in the 4e math?