((Posting after a party, so, if this is incomprehensible, blame the beer.
))
I think Mustrum Ridcully has the right of it. You can't really have it all. You have to choose between elements and whatever choice you make will restrict other elements. As Lanefan says, balancing classes by xp requirements is a possibility, although, in practice, I'm not sure that it really worked out all that well. At worst, you're a level behind the other PC's. If your class is more powerful than 1 level of difference, then the math is borked. That would be very, very difficult to balance I think. 1e, IME, did a very poor job of it.
To give an example, which I hope doesn't piss anyone off, because, honestly, I'm not trying to pick a fight here, look at mass combat rules in D&D. At one time, you had Battlesystem. A neat little minigame that was fairly decent at simulating a mass battle from a fairly high altitude. The PC's couldn't affect the outcome very much. But, it did do mass battle reasonably well.
Compare that to Heroes of Battle. In HoB, you have cinematic battle scenes where the battle between two forces (or more I suppose) is pretty much controlled by DM fiat. It's simply the backdrop for the PC's actions. The entire focus is on the PC's. So, the PC's can affect the outcome of the battle greatly, but, the rules are very poor at emulating a mass battle.
So, it comes down to a choice. Which one works for you? Now, I most emphatically, completely, totally, 100%, want to concretely state that whichever choice you make is NOT WRONG. There is NO WRONG CHOICE. I am NOT, 100% not, completely not, emphatically not, trying to tell people that if the choice they make is different from mine, that they are wrong. I AM NOT SAYING THAT.
There, with that disclaimer in place, hopefully I can forestall any further antagonism with people I have previously pissed off. Sorry I pissed you off in the past. But, I'm really not trying to piss you off this time. Honest.
Anyway, back to my point. The choice you make depends on what you want to gain. Do you want mechanics that detail the outcome of the battle, or do you want mechanics that detail the PC's interactions within the battle? You can't have both. THat doesn't work. Where you draw the line is entirely right for you. For me, I'd go with Heroes of Battle, because I don't care about what happens outside of the player's actions. The campaign is about the PC's IMO. So, mechanics that shift the focus away from the PC's are bad, for me.
YMMV and all that.
To me, I prefer mechanics that focus on game play at the table. Herreman the Wise has raised serious points about healing in 4ed. Fair enough. My response would be to shift the focus. In 3e, the DM could narrate wounds if he wanted to. In 4e, he can't. That's the point of giving narrative control to the players, in the form of Healing Surges. It becomes the player's problem. Have the player's narrate the effects of a serious wound. After all, it's the player in 4e who is going to determine whether or not the wound is in fact serious or not. If the player uses a healing surge to negate the wound, then it's up to the player to present a logical, reasonable representation of the action. The DM doesn't need to be involved.
