The designers already stated that they want the success rate to be about 50% at all levels.
However, I suspect that this means that when a Fighter adds all of his offensive Feats and Talents and Magic Items, etc., he will often be in the 70% range for melee whereas the Wizard will be in the 30% range for melee attacks.
For same level opponents.
The Ranger/Rogue might be in the 50% range for melee, but the Ranger might be in the 70% range for missile weapons. Hence, given a choice between melee and missile, the Ranger PC will often opt for missile attacks.
However, oppnents will typically not have the proliferation of magic items that PCs will have. Even if WotC minimizes the Christmas Tree effect, whatever ratio they have of magic items to level, the PCs will almost always have more magic items than NPCs. Otherwise the PCs will get the Christmas Tree effect.
So the PCs should, theoretically, have a higher chance to hit NPCs than NPCs have to hit PCs for same level NPCs.
With regard to armor, I'm hoping for a system where armor is actually subtracted from Reflex Defense, but adds in as DR. In heavy armor, one would be really easy to hit, but take hardly any damage.
For example:
AC = 10 + Dex + Level - 1/2 Armor
To Hit = Str + Level (i.e. BAB)
DR = Armor
So if Plate mail is Armor +8 like in 3.5, a same level opponent with same Str as the Fighter has Dex hits on a 6, but subtracts 8 points of damage from the damage. Sure, the Fighter gets hit a lot, but he often comes out of fights with a couple of scratches.
I'd also like to see DR allow for a minimum of 1 point of damage on every hit.