EDIT: I've removed the issue of Multi-targeting power users and the troublesome nature of ongoing damage from the lecture plan.
The prevailing attitude I see and hear about solo monsters in 4th edition is to "Never drop one into a battle and expect it to work on its own-use satellite monsters to liven up the fight."
Which is all well and good, but fundamentally defeats the entire premise of such a monster being a solo monster. A Solo monster is not just a bigger bag of HP and XP, it's an encounter in and of itself, and if the solution is to add additional monsters, you're subverting that intention.
Now, it's certainly true that many encounters can be dramatic utilizing a solo monster and some add on monsters-the standard "Floating head with a pair of disembodied hands" archetype is quite appealing. But then you also have the godly warrior who should be able to take on armies by himself, that the PCs wish to face off against-am I to make his right pinky treated like a minion? I think not.
Fundamentally, the fallacy being committed when it comes to solo monsters is thinking that one monster is five monsters. The errors that arise are the result of this thinking-overlooking the differences between one monster and five monsters.
I'll create a seperate entry for each of the following premises, but for now, I just want to get them on the table. You'll note that most of them fixate around targeting (though there's a surprise twist at the end):
1.) Defenders are overly effective. The most common way of subverting this is with close burst attacks, which tends to leave defenders feeling useless. More on the full dynamics of this later.
2.) Status effects are too powerful; stunning a monster for one round in a normal fight reduces enemy effectiveness by 20%. Stunning a solo monster for one round in a normal fight reduces enemy effectiveness by 100%! (roughly). The unique resistances to daze and stun are a band-aid to fix a gaping flesh wound.
3.) Action economy: This isn't just about damage, but about overall preparedness and ability to respond to battle.
4.) Solos don't die at the same rate as standard monsters, meaning that their damage per round is flat over the course of a battle, unlike standard monsters, which typically wildly reduce their effectiveness after the first round.
Like I said, I'll be addressing these points one by one in later posts, but for the moment, it seemed like a good idea to set out a decent guideline.