1) Action Points. In most cases, solos get two actions in the first two rounds of combat..that's a big bonus.
2) Area attacks. Solos generally have ways of hitting the whole party, or at least large chunks of it, with every attack.
If they can get to act, which brings me back to my initial point about the usefulness of action-denial. Wasn't there a thread some time back about a lv30 fight with the tarrasque, and the battle ending with the tarrasque being stun-locked over the course of 7 rounds and never managing to get in a single attack?
At lower lvs, this does not seem to be a problem, since PCs have so few powers available at their disposal. But at higher (especially epic) lvs, each player will have 4 encounter powers and dailies (and means of refreshing them). Assuming they stagger the order in which the disabling powers are used appropriately (eg: each round, 1 PC stuns/dazes etc, the other 4 attack, then swap responsibilities), it does not seem all that hard to keep the solo enemy disabled long enough for the rest to keep beating down on him.
To cite a more extreme example, a wizard gets confusion as a lv27 encounter power, and demigod30 lets him use it every round. So if the solo is not immune to fear (which interestingly enough, the tarrasque is indeed immune to fear), the wizard can just use confusion every round, effectively denying the enemy the ability to do anything but attack itself...
So it seems that even solos need the presence of mooks to help support it, such as helping it get combat advantage, taking some heat off it etc. I think 8 pit fiends would pose a much greater threat than orcus, simply by having 8 times as many actions....