Actually, I find that the current (0.86) beta provides some strong disincentives against all-hero stacks -- mostly that heroes tend to accumulate at a much slower rate than in EWOM due to a variety of factors (level, cost, etc.), and that it takes increasing amounts of time and research to bump your army size up by 1. If you try running around with all heroes, they tend to get killed on a regular basis (due to low levels of hit points) and then resurrected with various unbuffs (some minor, some not so). You start off with an army limit of 6 units -- you can bump it up to 7 pretty quick, but getting it to 8 takes a long time (unless you're willing to neglect lots of other critical research). I don't think I've ever gotten to an army size of 9 -- there have just been too many other techs I've needed to research.
That said, it's still possible to build stacks o' doom, but with a few heroes, rather than a lot. In the game I was playing last night (until 2:30 am), I had only three heroes for most of the game: the rest of the stack starts out as spearmen cannon fodder, but I push quickly to get to archers, and then to yeomen (my longbow units), and finally to mounted yeo: longbow units on wargs. I also put my heroes on wargs as soon as I can (wargs give slightly more initiative than horses). Initiative is critical: I focus on it for my custom sovereign (who is a fire mage), and it's an attribute in my yeoman designs (which have the Charge, Fast, and Accurate buffs). if I can get off a fireball (or a mana blast, for fire-resistant baddies) and three or four shortbow/longbow volleys before the other side makes contact or can shoot, the battle is pretty much over before it's begun.
That said, let me note with great reluctance that I think that the Mana Blast spell may be a bit too powerful. I say "with great reluctance" because I just discovered it last night, and it suddenly made life a whole lot easier. It costs 30 mana points up front to cast, then uses 10% of your mana point balance for the attack; if successful, it does that many points of damage; if unsuccessful, it does only half that damage. Since my custom sovereign is a mage, I tend to focus my cities and outposts on capturing shards. This means that towards the end of my playing last night, I was running a mana point balance of anywhere from 500 to 1500, with 15-20 mana points accumulating each turn. That, in turn, means I can cast a mana blast spell on a target (such as a boss) and do 50-150 points damage (25-75 if it "fails"). Oh, and unlike Fireball, it can be cast every turn. Woot!
Now, I think it's there because there are some big nasty bosses in some of the Quests. I beat one such boss (a walking mountain in the desert; its name escapes me) in a previous game, but lost everyone in my army except for my sovereign in the process. (Let me say in passing that I love the ongoing improvements to tactical combat.) The only reason we beat him was because all the units were on warg-back, and he was relatively slow moving, so it took him a bit longer to stomp on each unit, though one stomp was all it ever took to kill each unit.
By contrast, last night my stack took on the Burning Lands, which provided an addition disadvantage, since none of my fire spells would work. But both my sovereign and one of my heroes could cast Mana Blast, while another had a decent (level 5) longbow and some relevant buffs (dealing with initiative and accuracy); the other three units were just on-foot archers. The boss fight turned out to be against three fire elementals and seven ignyses (of varying strength). I believe we won without losing a single unit, though a few took damage.
Which brings us to an interesting tactical AI note. I saved the game before going into the battle (hey, I'm no dummy); I just now reloaded that save point, went into the battle, and put it on 'auto-play' (the computer plays for you). Neither my sovereign nor my (magical) hero used spells at all; they just used their weapons. They still won, but the two heroes fell, and two of the three archer units were killed (the third was damaged). So the tactical AI wasn't able or willing to use the Mana Blast spell.
Which, as far as I'm concerned, is a good thing. I would be very sad if I got into a battle with an opposing (AI) sovereign who promptly killed my sovereign with a mana blast or a fireball. Because, hey, that's what I'd do. 
..bruce..
