One of my rules of thumb for evaluating RPG combat systems is the number of times you have to roll to resolve an attempt to hit, on average. For example, in D&D, you have to roll twice — once to hit, and once for damage. In Vampire, you roll at least three and often four times — once to hit, maybe once to dodge, once for damage, and once for soak. In Feng Shui, you roll once — the roll to hit is also the roll for damage.
My assumption is that (assuming a standard combat system, rather than something more narrativist) fewer is better, because it make combat flow more quickly. There’s an orthagonal concern, which is getting the feel of combat right; for most games, you don’t want to say “roll 1d6 and if you get a 4 you hit, and if you get a 5 you hit and kill.” That’s quick and simple but most likely not satisfying.
However, I recently decided that this is too simplistic. While playing Mutants & Masterminds, I found myself getting all antsy about the combat system. Which is weird, because it’s simple: one roll to hit, one roll for defense.
But it’s a different person for each roll!
So, the addendum: you have to take information transfer into account. Go back to D&D. Roll to hit, tell your opponent what you rolled, roll for damage, tell your opponent what you rolled. Compare and contrast to M&M — roll to hit, tell your opponent what you rolled, tell your opponent what your Damage bonus is, your opponent rolls a Damage save, your opponent tells you if you did damage.
Same number of rolls, but you keep having to pass information back and forth. Feng Shui, the ruling champion of quick combat systems, is way simple: roll to hit, tell your opponent what you rolled, opponent tells you if you did damage. Hero is on par with D&D — roll to hit, tell your opponent what you rolled, roll for damage, tell your opponent what you rolled.
Of course, you really ought to figure in math complexity. It’s easier to do the math in Feng Shui than it is in Hero, and Hero is noticably more complex than D&D (since you’re counting BODY and STUN from the same roll, and adding a lot more dice).
The key observation, though, is that information transfer matters. I’ve heard more than one game designer talk about giving the defender a chance to roll to “involve him in the game” and so on, but I begin to think that’s a misguided concept.