You've got good points. But when the a very large proportion of the game *is* combat, they should have some juice.
Here's an example. 10th level wizard. Spellcasting prodigy, Spell Focus: Evocation, Combat Casting, Greater Spell Focus Evocation. Cast fireball, which he isn't high enough level to quicken, and thus he gets *one*. For 10d6. A maximum of 60 points of damage, only once a round. admitedly this can strike multiple opponents. But lets do the math. The mages Dc fore the fireball, as above, under current rules, is (with 20 int) 21. (+5 Int, +1 Spell Focus, +2 Greater Spell Focus, +Spellcasting Prodigy, +3 spell level) Note that even if he *could quicken it, the save would be the same. His Ac, if he's had time to buff, is impressive, about a twenty, assuming a 14 dex, Shield, and Mage armor. (two spells down) the mage has An average of thirty to forty hit points by this point.
10th level rogue. 20 dex. Weapon Finesse, Lightning Reflexes, Improved Crit (rapier). No items. He has Evasion, probably Improved Evasion, meaning he'll take half damage regardless. He has a +14 reflex save (+5 dex, +7 base, +2 feat), meaning he saves for *no damage* on a 6 or better. And he has more hit points than the wizard. He has threatens a crit with a rapier on a 15-20. (One fourth of the time ) His AC, again assuming no magic, is about 18. (+3 leather, +5 dex). His to hit with the rapier is +12/+7, still no magic or other feats. He hits an AC18 rogue on a 6 or better the first time. An AC 21 fighter (best possible AC without magic or exotic feats) he hits on a nine or better. Over half the time. If he manages a sneak attack (which isn't too difficult IME) he does 1d6 +1 (12 Str, say) +5d6. Or 7d6 per hit on a crit. So just hitting, not critting, the rogue can do 72 points of damage per round.
10th level fighter. Weapon Focus, Weapon Spec. Great Sword or great axe. Power attack. Str 20. Cleave. Great Cleave. Improved Critfull plate and heavy shield. He's AC 21, has a fort save of +10, +12 with Great Fort and Con 16. (He's still got feats left...) His reflex save is +4 or so. but he has 9 hp per level assuming average rolls, for about 90 hp.His to hit is +17/+12/+7, with masterwork weapon,weapon focus, and 20 str. Chances are good the fireball, even if he fails his save, just singes him (average damage 30 points). The rogue nails him for 36, maybe 72. He nails the rogue (on a 1/6/11)or the wizard (on a 3/8/13) for 2d6+10 with the sword, or 1d12 +10 with the axe. Easy hits. Up to 26 damage per hit, three hits, up to 78 damage. If he crits, that can go up as high as 156 points with the sword, or 234 with the axe. And crits are more common than one might expect. With no crit and avergae rolls, that's still 17 points per hit, 51 points per round. Add in the power attack, or magic, and it gets more hideous. Not to mention that if he gets to the mage, and hits him, the mage is casting *nothing* if he somehow isn't dead. (51 average damage vs 30 to 40 average hp, if the mage has a con bonus).
And the rogue and fighter can do their damage all day. Over and over again. As many combats as they can survive. themage runs out of spells. The more effecitve he is in combat (because of buffs and such) the less endurance he has, and he must rest more often to recover his spell list. Sometimes, (often) he must rest after one or two combats. And he probably hasn't memorized enough battle spells to deal out his damage every round of a ten round combat. The fighter will. The rogue might.
(1oth level mage as above has 4/6/6/4/4/3 spells. He's probably used two to three spells just to buff before each combat, if he wants to have the AC above. He has half his spell selection in utility. That leaves thirteen spells that could be combat spells, only some of which do the kind of damage mentioned above..and all of which are relatively easy to save against. Much more so when you add in save boosting magic. And he spent three of those spells to ready for combat. So he could conceiveably cast a spell every round. *2* of which might be fireball...Two of them are ray of frost (DC 17, 1d3 damage, and he has to roll to hit) two of them are magic missile (a danged good spell, up to 25 points of damage at this level either divided up or targeted on one enemy) 2 of them are like Ag's Scorcher (5 ' path, Ref save with the problems above, 5d6 damage max...roughly half as effective as fireball. Get rid of instakill, and 4th level direct damage spells are...Ice Storm for up to thrity points. Shout for up to thirty points. *less* damage than fireball. (Pure SRD here.)
5th level, no instakill, you have cone of cold, for, you guessed it, 10d6, in a larger but harder to use area. It can be devastating if the wizard uses another third level slot to fly, but again, only to large numbers of mooks. So. If you nerf instakill, your wizard will be reduced to crowd control or buffing (which the priest is better at) while your fighters and rogues (or monks) kill the BBEG's.
There isn't a really good answer. The system causes the problem. but we still like to play it.
