First of all, my apologies for all the lame titles-but also, my warning that they're here to stay.
The problem with status effects on solo monsters is pretty well recognized-so much so that Wizards of the Coast has already taken action! Well, problem solved-except not. The solution that Wizards took was, in order to negate the impact of two specific status effects (daze and stun), most solo monsters have a mechanism whereby daze and stun are less impacting than for other monsters.
I'm not sure how to feel about this. On the one hand, it's a deliberate fix that partially addresses the problem, and is certainly better than leaving things as they are-and if nothing else, it shows that Wizards at the very least understands that the framework for solo monsters has to be different from the framework for standards and elites.
But the problem with status effects isn't just daze or stun. The problem with status effects is that, most non-controller powers that drop any significant status effect (barring ongoing damage) is single target. When you let, say, Stunning Strike apply to "the entirety of" a solo monster, you're effectively letting the rogue target five creatures with this one power.
So why is this a problem? Well, let's look at a typical lineup at 17th level*: Fighter, Rogue, Wizard, Cleric, Warlock. All PHB heroes.
Each character has three encounter attacks**. Now, not every power can be a daze or a stun-but at least one power from every class sure can be. So, over the course of a standard five round encounter, you can expect creature-rounds of stun***. But if that's a solo monster, that means you're effectively getting 25 creature-rounds of stun! That's the entire fight.
But it's not specific to stunning. It applies equally well to weakening. And certainly, if the entire party focuses on weakening powers, then it's not entirely out of line to halve the damage of the enemy for the duration of the entire encounter (though I wouldn't necessarily suggest it's the right solution either). And if it's immobilizing the enemy and the enemy is all melee, you can certainly let your melee withdraw for those rounds, and turn it into "safety damage."
The point is, every status effect is valuable, and letting status effects intended to apply to one creature apply effectively to five creatures fundamentally alters the expectations of the game.
Of course, there's exceptions to this. When I use Icy Rays (Wiz3) or Freezing Bolts (Wiz23) I'm immobilizing two or three enemies, respectively. Against a solo monster, I don't get that advantage-it's one creature, all or nothing. But all is five (whereas nothing is still nothing). So, whatever solution we choose to integrate has to take into account the controller's already disadvantaged state-after all, when it comes to solo monsters, overpowered status effects are all they have.
The situation is complicated further by the fact that "status effect" really isn't one single well defined concept. After all, solo monsters being one monster that equals five plays into a lot of powers in unexpected ways. The durability of a "The entire party gets +6 to attack and damage against the targeted creature" is much higher against a solo monster than it is against a standard; for a standard, the chances of the party taking down that creature and, as thus, "wasting" the remaining duration (end of next turn, end of encounter, save ends, whatever) is pretty high. But in the event of a solo monster, it is as if the remaining duration "transfers" over to the next monster. This is a pretty staggering fact! But on the other hand, I don't think the game developers were necessarily unaware of that fact-indeed, I'd find it pretty unlikely that anyone would expect you to use an End of Encounter party buff vs target daily resource against a standard monster
On the other hand, I've seen too many fights that essentially boil down "Daily power saves the day." Some powers aren't overwhelming in the generic scenario where they apply to standards, but you never use them in those scenarios, so they can't be evaluated in those scenarios. So we can either measure how these powers function in the scenarios where they're reserved for, or we can try and address solo monsters.
One potential option is the idea of a "Cleanser" ability. A solo monster at certain times in a fight (probably no more than twice, and usually no more than once) would have the ability to cast off any status effects affecting it. How this is written is pretty important though-does it just remove conditions, or does it fundamentally make the monster treat itself as an entirely new token on the field, to which no earlier effects apply? I'm cautious of this approach, simply because it requires a lot of ad-hoc intervention to make any sense at all.
Another option is to grant solo monsters the ability to save against "short term" conditions at the start of their turn. 1d20+5 means that a solo monster has an 80% chance of success on any such roll, so an end of turn effect has only a 20% chance of actually affecting the creature's turn. From a simple math standpoint, that answers the question of End of Next Turn (or ENT) effects, but it leaves something to be desired from the standpoint of rewarding player actions. Furthermore, if mark powers grant conditions to the target, then if the target also has the ability to shed marks (see part 1 of Solos), you're double penalizing defenders. Fortunately, this is almost never relevant-mark powers typically trigger on the triggering creature's turn, meaning that they'd almost always end before the start of the creature's next turn anyway.
Of course, this style of rule, like all End of Turn Start of Turn rules, tends to have problem with what I like to call "Turn Shenanigans." That is, readying an action for after a creature's start of turn, such that they cannot make a save against it for a full round. This kind of behavior requires DM Ad Hoc intervention-I don't have any way around it other than to say "You want to play that way?" and instigate "Mutually Assured Destruction Mode."
Of course, there are two things to remember. One: Solo monsters already get a bonus to saving throws. A standard creature has a 55% chance of passing a save, whereas a Solo has an 80% chance of passing a saving throw; this translates into a 45% chance of failure (extend duration 1 round) for a standard, and 20% chance for a solo monster. Assuming an effect is purely save ends, it lasts for one round + X + X^2 + X^3, and so on (I'm not good with the calculus but it's relatively easy values). In short, a typical save ends effect lasts for approximately 1.8 rounds against a standard monster, and about 1.25 rounds against a solo monster.
In other words, save your single target save ends effects for solo monsters, because you're getting 6.25 creature-rounds worth of status effects for every one-that's three and a half times as much as against a standard monster!
For once, I see no problem with this. Single target save ends powers scream "Use this against a solo monster." And it's certainly true that, no matter what your deadly Level +4 solo monster boss fight is, if the entire party loads, one by one, save ends stuns and you decided that you wouldn't include a "still effective while stunned" property because you figured dealing with End of Next Turn effects was enough, your boss will never get a turn-barring crappy hit rolls. But ... actually, I have no retort for that. Use the cleanser rules too.
The danger, as I alluded to earlier, is of players feeling like their options don't count. No one really wants to bank on the 1 in 5 chance that their end of next turn power will really work. And no one wants to see their daily powers go up in smoke due to a 1/encounter status cleanser. Players will feel cheated. Implementing changes without negating player input is important. But so is challenging characters. And if the only solution to challenge characters is to simply not run solo monsters, then solo monsters really shouldn't be a part of the game.
Whatever mechanism is used, controllers need to be compensated somewhat. Either a bonus to hit or damage, or a penalty to enemy saving throws. It's an ad hoc solution, but I wouldn't be opposed to having controller powers grant a -5 to the start of turn saving throw for solo monsters.
Obviously, when running against an entire party of thieves and scouts, it's a non-issue. If the group uses straight DPS, you don't have status effects to worry about. But that's of little consolation when your group deals 700 damage over the course of two rounds at 16th level!
*17th level is when Wizards get Ice Tomb and Phantasmal Horror, their first encounter stuns, so it's a convenient level to use.
**We're ignoring paragon path powers for the moment, but trust me-they make the model worse.