There seem to be two issues for the loudest whiners here. First, characters with good armor saves make GREAT tanks and can keep absorbing hits. Secondly, characters in multi-wound units or character units with multiple wounds have a sort of wound allocation akin to 5th edition - most people hated this and wanted to see it gone...yet there it is. Let's be clear here though, this is not as good as it was before - now you're relying on dice to allow you to transfer wounds, your units have to be a bit closer together to make sure everyone can LoS to each other and your opponent can move around 2+ LoS! characters, etc. Yes, units can reach you wounded but fighting fit but it is far less likely. So, let's look at how one can actually use these advantages and then how one can counter them.
Let's look at the first - tanking characters. Any character with a good save and good amounts of wounds is a great tank. And not just because of the obvious. The obvious is, a character often has better armor than the plebs of a squad and by sticking his chest out front, he can use said better armor save to protect the squad. When anything big comes along which can instant death him or bypass his save, he can LoS! it to one of the plebs in the squad. This effectively gives the squad a better armor save from shots coming in from the front. It also ensures the squad is less likely to lose potential charge range distance by allowing members from the back of the squad to be picked off when necessary (through LoS! rolls) rather than the front (as would be usual with the new model removal rules). However, it also increases the chances of the character to be reduced to ash or simply become terribly inefficient in combat (i.e. 1 wound left) where they get challenged and die (or refuse and thus take no part in the fight). LoS! rolls aren't a 100% guaranteed thing either, a 2+ is a 2+ but enough dice and that 2+ fails or enough dice on the character to roll saves and he'll die as well. That being said, a cheapie throwaway character with just a good armor stat could go a long way (rather than say an expensive beatstick who if he dies or gets significantly wounded, hurts the unit).
Characters can also be used to tank hits in close combat assuming they are not involved in a challenge. Ensure they are in base to base and using the mixed armor rules, can take as many of the hits until they die or can use LoS! to allocate the wounds to members of the squad which are not going to become engaged (thereby saving attacks if the squad's initiative step has yet to be reached). The Baron + Eldrad with a 2++ and 3++ respectively + Fortune could make a very potent combination here...if only they had a good combat unit as well (Banshees w/Mauls =D!). Between the two, this is much more powerful compared to a characer tanking shooting as its a lot harder for your opponent to avoid.
Which brings us (somehow) to the 2nd point of mutli-wound character units and wound allocation. A character in these units will make all of this much, much easier, 2+ > 4+ after all. But the character nature of the squad also ensures if the character is bypassed, they still have LoS! options. This allows the unit to spread wounds as much as possible, much like in 5th edition and take wounds but maintain fighting effectiveness. Unlike 5th edition, this is MUCH less reliable (50% compared to 100%) unless there is a character around using his improved LoS! roll. There are often models you want to keep alive in a certain squad (i.e. Painboy, PK Nobz in a Nob Biker unit) and these will need to be kept at the back/centre of a squad. An enemy is going to be able to bypass a single character at the front of the unit (unless they are a static gunline, i.e. bad army) or the character unit will fail enough LoS! rolls and then saves to kill off the front models. This reduces the effective threat range of the unit but ensures the key models are more likely to stay alive. Again, worse than 5th edition by a fair margin but allows such units to have some method of wound saturation.
On the other hand, not all is a bed of roses and there are a lot things you can do to counteract this so-called overpower tactic. The most basic way to get around characters tanking for units is to move around the character. MVB did an amusingly little diagram which I'll post here...
This is an extreme example of 6th edition being uncimenatic (bending bullets) but the concept is the important part. All you have to do is make sure the unit shooting has a model closer to a model which isn't the character, preferably closer to multiple models which are not the character (you don't want to kill one pleb and then the character go back to tanking after all). You don't have to be this close either, move to the side or angles of units to ensure the character simply isn't the closest model and you can bypass their tanking ability. Put a tank in front of the character so you cannot see the character. Woops, can't shoot it now, have to shoot the unit. This is basically the opposite of sniping, you want to avoid the character. You can further this to snipe specific models out of said unit as well - just make sure the character is hidden. Against character units they still have the option of LoS! rolls back to the character or away from the model you are attempting to snipe but you're forcing them onto a 4+ LoS! roll, much better than a 2+ and you're more likely to damage the unit rather than the character itself. This is the goal.
This is harder in close combat if the character has a better save (and thus we are using mixed armor allocation rules). Even challenging isn't going to stop this as if they are base to base contact, they can refuse and remain there, they simply don't get to attack or use their Leadership. The best way to defeat this then is during your movement phase. Use other units/vehicles to block the pile-in moves of the character or to completely push them out of the way (tank shocks - though be careful, you can force the unit to flee here which can be disadvantageous at times). String your unit out so it's less likely to engage the character during their pile-in moves (though be careful you don't leave yourself vulnerable to being overwatched out of charge range) or move the unit so you're charging from a different angle which keeps the character out of it. The important part here is to engage and defeat the unit in one round so the character doesn't get multiple pile-in's to begin tanking for the squad. Otherwise, you want to engage and keep engaging the unit forever with a much cheaper unit (i.e. fearless Orks, Termagants, etc.).
Use your noggin to get around the character tanking - if you throw yourself (or your shooting) straight at the unit, the character is doing what it was designed to do and unless you get lucky, you're going to break your army on that rock and the rock will subsequently crush you. Tactics and thinking are what defeats the extreme versions of character tanking (aka deathstars).
In comparison to 5th edition wound allocation, LoS! actually requires some thought to gain the advantages and there are a few other applications which couldn't be done previously. At the same time, as an opponent facing this, there are options around this and wound allocation in general isn't as powerful as it was before for multi-wound units. In general, more thinking about model placement and movement is going to be paramount to both get the best out of this rule and minimise its impact. And more thinking is never a bad thing, is it?