EDIT: I'm lazy, so rather than trim this down, I created an abridged version of the post that cuts through all the justification and just gets right to the conclusion.
In your standard encounter, the defender will typically not be able to mark all enemies. There are exceptions to this, but if you take a look at defenders in general, it doesn't appear that it's really intended that everything on the field be marked.
It's true that marked enemies typically have to make the decision between suffering the effect of the mark or targeting the defender. But, when have you ever run into a group of five monsters where every single one of them simply ganked the defender? Actually, the play experiences out there are so wild and varied it wouldn't surprise me but, again, and perhaps this is my own faulty intuition, this is not the default case.
Generally, defenders are a very poor choice of target if not for their defender features. In terms of reducing net enemy damage per round (as in, reducing HP causes offense to go to 0), they're less than strikers, and defenders-knowing full well that they're built for defending-tend to also have high defenses. So, given an even choice between attacking the defender and attacking, say, the leader, any given monster who understands the rules in play is going to go after the leader. Assuming the leader doesn't have anti-targeting mechanisms of his own.
But defenders can't successfully control the entire enemy party all at once-even if they have everyone marked, they only have one immediate action per round. So while that -2 to attacks may be ever-present, the decision making process for the monsters is different before the defender's immediate action, rather than after wards.
But wait, there's more! We can even get into the game theory of attacks now! If a soldier knows that by triggering the defender's immediate, he knows the defender can't use it against the artillery, and the soldier knows full well that the immediate is less threatening to his side's overall position when used against himself, rather than when used against the artillery, then he can subvert the mark freely. The defender, knowing that using the immediate on the soldier would be a waste-and free the artillery to attack others, decides to reserve that immediate action. Thus, it doesn't matter how many targets the defender has marked-the only one that's actually going to be redirected to him is the one who the mark's effect would be most significant against. In a very real sense, the defender is only marking one creature at a time, even if his mark is multi-targeted.
With solos, you don't have this dynamic. WHERE the enemy is when the mark triggers can vary, as well as when, but ultimately, when it comes to a solo it's not a question of who is marked, but a question of whether the enemy force is marked or not.
Obviously, there are those that see this as a feature. I'm not so certain. In practice, solo monsters have four main modes:
1.) Heavy damage: That is to say, rather than making one melee attack per round, they make five! In this instance, you run into what I like to call "Lobbing nuclear weapons." With all the focus on himself, the defender's chances of surviving two consecutive rounds without healing drop pretty substantially, whereas the surge count of the rest of the party remains largely unharassed.
2.) Single target attacks that are really multi-target attacks: Trample is the most common feature we have here, though Hand of Radiance style attacks function the same. The issue with this kind of mode is that its effectiveness varies wildly from defender to defender. Fighters (assuming you can't exploit Combat Superiority!) and Assault Swordmages are essentially converted into just another striker, whereas Shielding Swordmages are leaders who don't require you to use a surge. Wardens and Ensnarement Swordmages can either completely negate this mode by totally interrupting the movement (either by slowing the enemy or by simply relocating them). Battleminds on the other hand can be rendered either next to useless (unless they have a good opportunity attack) or unfathomably powerful, depending on the circumstances. These distinctions between the classes are, when looked at in that context, a good thing. However, it just goes to show the unpredictability of the game engine when making small changes.
3.) Close and Area attacks. This tends to be my preferred way of building solo monsters-particularly of the "Bonus action on Initiative Count +10" style, but I tend to hear that defenders forced into combat with such solo monsters feel like they're not really marking the enemy at all. It's true that if every single attack targets every single PC, it doesn't matter that the creature is marked. Still, control is an element-the smaller the burst, the more control the defender has. If the defender is physically distant from his allies, attacking the defender and said allies as part of the same attack becomes a non-option. Thus, again, ranged defenders (Shielding and Ensnarement swordmages, along with some paladin builds) can be fundamentally more potent in this regard than melee defenders-particularly if a given party is all or predominantly melee oriented.
4.) Non-Attack Options. This is the one that I feel gets the most mileage, but also the one most groanworthy. The DM in my current game has a fascination for damaging auras, but quite often doesn't give an option for not suffering the aura damage (one specific encounter involved something along the lines of 35 lightning damage per round, or 100 if you weren't in the aura). Likewise, with ongoing damage, when used in conjunction with close or area attacks, the initial damage may not be relevant, but the ongoing damage will be. Finally, most marks trigger only on a successful attack-meaning that solo monsters with miss effects can potentially be better off missing with melee and ranged attacks than they are hitting! Again, this varies defender to defender.
What bothers me the most about all this is that the thing that's the most disorienting is that the most troubling scenario is also the most common-a fighter against a non-teleporting solo. Even ignoring Combat Challenge, the promise of Combat Superiority means that a fighter who previously could only keep a small fraction of enemies exactly where he wanted them can essentially completely immobilize the entire enemy force.
Then again, there's always shifting and charging-which turns the Fighter into a makeshift striker. Only downside there is with such feats as Mobile Challenge, allowing the fighter to have his cake and eat it too-the enemy triggers combat challenge by shifting, but then doesn't gain the benefits of shifting, as the fighter can still take his movement denying opportunity action , thus negating any attempts to charge.
But, Combat Superiority is part of the core of 4th edition. Mobile Challenge is not. If a DM feels that the dynamic of fighters is oppressive due to Mobile Challenge, that's a simple fix.
To put it another way, I fear I've digressed-
So ultimately, the question boils down to what the problem is. Ultimately, I view it as twofold: Either A.) four fifths of the party doesn't have to care about defense at all (barring using party buffs from a cleric) or B.) The defender is useless due to close attacks. The solution of automatic damage may do to add challenge to the game, but it's not particularly engaging. There's no decision making process involved or real way to utilize resources to interact meaningfully with the mechanism, except to drink a potion of resistance and say "Well, there goes a healing surge."
In all honesty, I do not have a solution ready. Every plan I have is morbidly complicated, involving various "tiers" of marking features that acknowledge that a "Mark all creatures" type effect is of greater effectiveness than a "Mark one creature" effect. And that doesn't even solve the fundamental problem of the lack of game theory a solo monster has that a standard does not.
So, why bring it up? Because like most things in life, I think the best solution is an imperfect one.
Solo monsters don't have the decision making process of whether or not to eat the mark or pass it on to their buddy-but there's no reason a monster can't have a substitute process. Namely, counter-mark powers.
Excluding the hybrid Swordmage, defenders don't typically have encounter limitations on marks. They may have encounter limitations on bonus marks, but typically, a defender can mark another creature and only lose the fact that he'd marked an earlier creature. But when the defender can mark a creature is usually quite simple: On his turn, when he has the actions necessary to do so.
A solo monster bequeathed with the unique ability to Shed the Mark (so to speak) on a limited basis does not make the decision simply to respect or subvert the mark of the defender, but also makes the decision of when he has to make that decision. That is, the monster is not deciding to shift the burden onto an ally, but rather, deciding when is the least disadvantageous time to be marked.
To deal with defenders, all monsters should have at least some mechanism of attack that does not simply target all enemies on the field. To the same extent, all monsters should have some mechanism to subvert defenders-to be able to make meaningful decisions of how they want to play their odds. This comes with a cost: either a trigger cost (the monster must be hidden, or the monster must self-inflict a certain amount of damage, or the party must have made some kind of specific mistake) or simply as a non-at will power (Encounter, Recharge when Bloodied, Recharge X, Recharge when a player says "Bazinga" whatever). Due to the nature of action economy, even a minor action cost on an at-will power may make for a viable tradeoff, but I'd strongly recommend against using it for anything other than monsters who already have plenty of minor action options each round. After all, if the minor action would normally go to waste anyway, it's not really a cost, now is it?
So, next time you play a solo monster, try it out-use a mark negation technique. Your defender probably won't thank you for it. And if he happens to be a hybrid swordmage, tell him to be thankful he still gets to be a striker.