Doug McCrae said:
The vast majority of monsters could be elites or solos. They're monsters, they're supposed to be dangerous. Even a kobold, provided it's an unusually tough and/or skilled one.
They could, but that would go against the group-vs-group thing 4e is going for. Most encounters are supposed to be a group of PCs against a group of monsters, with each group supporting one another and covering for one another's weaknesses. Just like the fighter in the PC party is there to keep the monsters from getting to the squishy wizard and to let the rogue flank and do sneak attack damage, the mind flayer has thralls to run interference.
A very important thing to remember is the whole "economy of actions" thing. 5 PCs get five sets of actions per round. In order for a monster to take up multiple monster "slots", it shouldn't just be big and tough (that's more a matter of its level), it should also be able to multitask. I think most elite opponents we've seen have had the ability to do multiple attacks, for example.
And if you look at the dragon from D&DXP, it had:
* Double attack (two claw attacks)
* Tail slash (immediate reaction, when a melee attack misses the dragon)
* Breath weapon (AoE attack, recharge)
* Bloodied Breath (when first bloodied, it gets an instant use of its breath weapon)
* Frightful presence (AoE stun followed by -2 to hit (save ends))
* +5 to saves (to keep it from being incapacitated by debuffs for long) and 2 action points (to get additional actions at critical points) - pretty sure those two are standard for solos.
What it did not have was super-high damage (a mere 1d4+3 per claw, or d6+3 plus ongoing 5 acid for the bite).