Ok, this is something in d20 that always bugged me. I can't change the rules for it in AORP (at least, not without some notice), but I want to hear what other people think.
Casting a spell provokes an attack of opportunity (from now on, AOO).
However, the spell is only disrupted if a) the attack hits you and b) you fail a save against a save of 10+spell level + damage (in hp).
Now, at first level, it's safe to assume that the average damage from an attack will be around 4, I think, but for the purposes of this example, we'll use 6, to make it more difficult. So your average difficulty is going to be 17.
A spellcaster with skill focus (concentration) and max ranks, plus a healthy Con (not unreasonable, it'd be the second or third most important attribute for most casters) of 14, could have a plus 8 to this roll at first level. which means he has a better than 50% chance of beating the difficulty, needing to roll a 9 or above for success.
It's even less of a threat if you have the Combat Casting feat (and human spellcasters can easily have both feats at first level), which gives you another +4 for Defensive casting. In this case, the DC is 15+spell level. Which already gives you a better chance, figure in the Combat casting, and you only need to roll a 4 or better to cast a first level spell, no matter what foe you face.
In other words, I don't think there's a significant threat to a spell caster who just stands there casting anything he wants in the middle of combat. Even if he's facing something that can do lots of damage. He has a better chance of doing damage and no worries that he'll lose spells.
Probably this doesn't bother most of you, but I like magic to be difficult to the flavor of the campaign. Probably I'll be removing the ability to cast defensively in future campaigns.
