The sweet spot and fixing the magic system [PEACH]

[Belial_the_Leveler]

Spellcasting
All casters have a number of spell slots depending on their casting ability modifiers. The level of each spell slot depends on their class level. All casters have simple, straightforward spells they can cast with a bit of effort and more complex spells they can cast after significant preparation and with a lot of energy.
Spell DC is 10+slot level+ability modifier. To cast a spell of a given level in a given slot the caster needs to have an ability score of 10+twice the spell's level.
Spells known, spellbooks and other spell availability remains the same. Spell slots are changed as follows;

Wizard
A wizard has a number of major spell slots equal to his intelligence modifier. They can be used to memorize any spell the wizard knows of a level equal to half the wizard's level, round up. A wizard recovers 1 major slot per full hour of rest or inaction. Slots used in the last 12 hours cannot be recovered.
A wizard has a number of minor spell slots equal to either his wisdom or his charisma modifier (whichever is higher). They can be used to memorize any spell of a descriptor the wizard has mastered. A wizard masters a single descriptor at 1st level, and additional descriptors at 6th and 12th levels. Minor spell slots can be recovered with a 5-minute rest per slot. Available descriptors are;
Damage: a spell that only or primarily deals damage
Boost: a spell that provides an enhancement, resistance or morale bonus. Cast from minor slots such spells last 1 round/level regardless of normal duration.
Summon: a spell that summons one or more creatures. Cast from minor slots, summoned creatures cannot use spell-like abilities or spells.
Control: any mind-affecting spell. Cast from minor slots, control spells allow a save every round and last 1 round/level, similar to hold person.
Curse: a spell that deals energy drain, ability damage, ability drain or bestows a curse. Cast from minor slots, effects last no more than 1 round/level.
Defense: any spell giving armor, shield, damage resistance or damage reduction. Cast from minor slots, effects last no more than 1 round/level.
Mobility: any spell increasing speed, giving additional method of movement or providing teleportation no further than dimension door. Cast from minor slots, effects last no more than 1 round/level.
Antimagic: any spell that dispels, counters, confers immunity or blocks spells rather than descriptors.

Cleric
As per wizard, except clerics use wisdom for major slots and either intelligence or charisma for minor slots. Their list of available descriptors for minor slots includes the "healing" descriptor; any spells that recover HP, ability damage/drain, negative levels, paralysis, poison and disease. In addition to the descriptors, cleric minor spells must belong to the cleric's deity's domains (the deity's domains, not the cleric's).

Sorceror
As per wizard, except sorcerors cast spontaneously - but only from their limited list of spells known. A sorceror uses charisma for their minor slots, and any mental ability they want for the major slots.

[Belial_the_Leveler]

The reason for the above changes are as follows;

Major spell slots
Casters should not have access to enough problem-solving spells to deal with every situation. The wizard wants to hide behind a Foresight and Ghostform and Superior Invisibility and Energy Immunities? Well, they can. But they just used up all their major slots so no Timestop or Celerity or Gates or Teleport. The reverse is also true; a wizard that wants to keep trump spells at hand won't have enough spell slots to also do the more important buffs so they'll be vulnerable. That forces casters to think tactically without banning anything outright.

Minor spell slots
Because no caster should ever be reduced to casting a single magic missile per day; even low-level casters can have fun. And casters will now need to think tactically; they are only going to have a limited number of the less fight-ending spells in a given fight. Even a 20th level caster shouldn't have half a hundred low-level spells to burn in a given fight, with those low-level spells including things like Teleport or Plane Shift that are outright safety nets, or contingency or any other trick that allows them to overcome the dangers of combat.

Higher ability reliance
Yes, a caster now needs an ability score of 28 to get 9th level spells, as opposed to 20. Because 1st level characters shouldn't be capable of understanding 9th level spells, even if they can't cast them. Also, wizards, clerics and druids now need two primary abilities to reach the pinnacle of their power, just like a decent melee character needs both strength and constitution. Spellcasters have a harder time optimizing spell DCs while also buffing up their defenses.

[Glimbur]

So it's a significant hit to spells/day, as well as making it much harder to cast mid to high level spells. Level 3 or so spells are still very feasible, but it's nontrivial to get to 20 Int by 10th level to cast 5th level spells. Yes, you can get there with racial mods or ability boosts, but it means that a caster has to be really dedicated. This hurts gishes, rogue wizards, rangers, paladins, and other partial casters.

It also doesn't actually fix the core problem of spells being too good. A spellcaster still has enough juice in the tank to make a larger difference in a fight or to break plots. Knock is probably not worth learning anymore, but Scrying and Speak with Dead and Teleport and Gate and Color Spray/Sleep and Haste and all those other spells that Do It Better (TM) are still available and still worth casting. This doesn't end Scry and Die, which is a problem for some game types. It does make it harder to layer on lots of spells for defense, but one response to that is to alpha strike harder. Metamagic up some orbs (in minor slots), use major slots for a few key buffs and Teleport, and go to town. Scrolls are still an option, as are staves.

You did throw a bone to blasters with the minor spell slots, and with a modest Wis modifier of +3 or so they can blast in most combats for most rounds.

The way I see it, the spells themselves are the problem. If you want to fix casting, you have to fix or restrict access to the spells. Warmage, beguiler, and so on are one way to do this. Caster replacements, like Binder, Incarnate, Shadowcaster, and Truenamer are (in theory) another way to do this, though their viability varies.

[Belial_the_Leveler]

1) Gishes and secondary casters don't have an issue. Getting access to 5th level spell slots requires an ability score of 20, meaning a score of 14 plus a +6 magic item costing 32.000 gp. Since treasure by 20th level is 800.000 gp, I see no problem. Gishes aren't going to get high-level slots early after all.

2) Which 10 spells is your 20th level wizard going to memorize? Whatever they do, they can't solve all problems any more. For example, a scry-and-die attempt requires 2 teleports (to get there and back), minimum 1 scry, a way to act first when you get there, a way to unload enough spells in a short time, and the several offensive spells to kill the bad guy outright. That's your entire complement of daily spells to off a single enemy. The other 3-4 enemies are up to the rest of the group, assuming the enemy you tracked is a single creature for the challenge, assuming all your spells succeed (lead-lined cloaks are cheap) and assuming you don't die in the attempt due to bad luck.

3) If you want balanced spells, use the Pathfinder's spell list. 99% of the broken stuff are gone. And contrary to popular belief, Gate is not an "I win" button. That Balor you called is smart enough not to use its telepathy on you when it arrives. Since it can't understand your commands, you can't command it not to beat you up nonlethally then feast on your flesh and soul while you're still alive. That's pitfall #37 with Gate. Care to try your luck with the rest?

4) Metamagic spells still take higher level slots. Higher level slots also require high ability modifiers. Also, Orb spells are so dangerous only because wizards can buff themselves to always hit with them, to win initiative and hit first and to take extra actions to hit multiple times. Make the buffs not so easily available and suddenly the offensive spells drop in power.
Or, you know, use Pathfinder. No Orbs there.

[Thrice Dead Cat]

1) With these rules in place, gishes and secondary casters do have a problem. Unless you have something in place that says otherwise, but then what's stopping the full casters from dipping into a noncasting class for a level to get the the same benefits? Human Paragon suddenly becomes a lot better.

2) Scry and die works on the basis that once they're dead (and you'll know if they have allies nearby when you scry, fyi), you don't necessarily need to get out right away. Scrolls still exist, too, don't they?

3) Who calls Balors with gate? The go-to buddy is a Solar, since they tend to be good to the point of cleave and smite. At low levels, grease may have been nerfed, but color spray and sleep still do the job perfectly well in PF. Glitterdust still blinds chumps, too.

4) Incantatrix, Spelldancer, and metamagic rods still exist, so, no, metamagic feats don't necessarily increase the spell level. Plus, with Echoing Spell, you'll never run out of spells.