I don't think you can't simplify combat in this game to merely being an economy of actions. It is much more complex than that. This is thinking of combat in D&D as more of a boardgame, where everyone's turns should be perfectly fair and balanced, rather than a roleplaying game, where there are many other factors to consider. It's not just an economy of actions, it's an economy where actions are only one of many commodities. Encounter and daily powers are also very important commodities in 4th edition. An ability which grants a player extra actions isn't necessarily unbalanced if he had to pay a daily power to get it, for example. There's also exp and monetary costs, ritual costs, etc.
And I think the most important thing to consider is the roleplaying "cost." People in real life can raise armies, build-up nations, or lead masses into religious observance. Why shouldn't an epic fantasy hero be able to do such things? Certainly it shouldn't be easy, and alot of it depends on the style of game you're running. But hey, if your player manages to convince someone to follow you through roleplaying, or manages to tame and train a dire bear, why should you be no better off for it? Why should that henchmen or pet not be able to fight for you?
I think a large part of the problem with minions in 3rd edition was that it was tied to class abilities (or the leadership feat). The two biggest problems with this is that only some people had access to minions and that they were guaranteed. If everyone has the opportunity to get minions, rather than just druids, rangers, wizards, etc, the so-called economy of actions remains intact. Fighters and Warlords can recruit cohorts, Clerics can call upon angels, Druids can get all their furry little friends, and Wizards can enslave elementals or demons to serve them. If everyone can get minions, then everyone is happy. There isn't a balance issue when everyone has the opportunity to get followers. But it should be just that, an opportunity, not a handout or class feature. If getting an animal companion is a class feature, than clearly all memebers of that class will have one. If they don't, they're gimping themselves.
I think companions should be mostly a roleplaying thing. If you want a dire bear, go out and find one, tame it and spend the time to train it. If you want cohorts, go do what is necessary in roleplaying to obtain them. These things should never, IMO, be tied to a feat or class feature. If a player spends a feat on leadership, he then has good reason to feel entitled to those servants. They are, effectively, part of his character. Likewise with an animal companion, familiar, etc. And then we often hand-wave away the roleplaying aspect. Those followers just appear out of thin air, or you can just pray for a new animal companion if it dies, rather than having to go to the work to get a new one. Having companions as a class feature makes them an entitlement, rather than something you have to work for. And doing that removes what is perhaps the best way to balance them.
I don't think having minions should be a default assumption. They can and should have drawbacks. Pets have to be cared for, and especially large and powerful ones, like bears, aren't always able to accompany their masters. They may not fit in the narrow hallway or be able to cross the rickety bridge. My group came across these situations quite often. With followers, there are logistic and morale concerns. How do you feed your army? How do you keep them loyal? How do you get them to accompany you on hellish adventures? Even the most disciplined fighting fource will be reluctant to follow a hero into one of the nine hells on some suicidal quest. And summoned creatures, which would be the most powerful minions, could carry other risks, such as breaking free of the summoner's control. Just as obtaining minions should be, IMO, a roleplaying concern, maintaining them is also something that should require effort. Not to the point that they aren't worth having, but enough that they aren't taken for granted.
I am against minions simply giving bonuses to their master. This is just too unbelievable for my tastes. A necromancer raises the dead to fight for him, not give him a +X bonus to his own attacks. That said, a necromancer class could still be balanced, especially if his own spells were weaker then Wizard spells. Perhaps the zombies he calls to crawl up out of the ground follow the minion rules, and die in one hit. I would prefer this to apply to temporary zombies created by an encounter/daily power, rather than those created by rituals, however. Ritually animated dead should be alot tougher, but this can be balanced by the cost of the ritual to create them.
I'm not necessarily against the idea of concentration being required to control summoned minions. However, I think that this should only apply to creatures summoned with an encounter/daily power, rather than long-term summons conjured by a ritual. I could certainly envision a daily power that allows a Wizard to summon an elemental for an encounter, but he has to spend his standard action to sustain it. Such an elemental should be quite powerful, though. If he has to give up his own actions, and a daily power, the minion should be quite impressive.
Ritual minions shouldn't require any action on the part of the master to control, save perhaps a minor action to direct them in combat. This could be balanced because you have to pay for this in the cost of the ritual, take the risk that the creature may break free and eat you if you don't overcome its will during the summoning process, and also the fact that the creature won't likely be too happy to be enslaved by magic, and may later seek vengeance. Sure, go ahead and summon that Pit Fiend. Just be prepared for the terrible revenge it will certainly seek after its term of service is up, and that's if you manage to subjugate it in the first place.