Sigh...
I was not trying to convince you two, but to have opinions from potential other peoples.
Just for these potential other peoples, I'll write a little thing in the defense of these things.
I'm not wanting to remake the magic-user class. If I were, I would work to adapt the Magus of Ars Magica.
For the "exclusive class ability", I'm not sure preparation is that exclusive. Prestige classes who prepare spells are a-plenty, and all divine casters prepare also. From a spell-list made of all existing spells, since divine casters don't need to bother with a spellbook, their god being the spellbook.
I found that critic akin to saying that the Weapon Proficiency and Armor Proficiency feats are overpowered, as they are giving the class abilities of fighters.
A wizard with the Light Armor Proficiency, Medium Armor Proficiency, and Martial Weapon Proficiency: Greatsword can be a weak fighter without sacrificing levels in the fighter class that would have slowed his wizard progression. A wizard with the Spell Mastery feat and a couple Signature Spells could be a weak sorcerer without having to sacrifice wizard levels. In the same way, the GAP feat allow a sorcerer to be a weak wizard by spending feats instead of levels.
Gamecat pleasantly said it was broken from start and will always be. I imagine that, if I modified the feat in this way, he would continue to find it so:
Greater Arcane Preparation v2.0[General]
You are a bit learned in the way of wizardry.
Prerequisite: Extra Slot, Scribe Scroll, capacity to cast arcane spells spontaneously, capacity to prepare arcane spells (either with a level of wizard or similar class or with the Arcane Preparation feat).
Benefit: Choose a spell slot of a level no higher than your Intelligence bonus.
That spell slot must be one you gained through the Extra Slot feat. You can now use this spell slot to prepare spells from spellbooks just as a wizard does.
You can now read and write spellbooks just like a wizard. You can also take the Spell Mastery feat just like a wizard.
Spells known (as a sorcerer, bard, or similarly casting class) are considered mastered and can be prepared without spellbook. However, spells mastered with the Spell Mastery feat are not considered known spells for spontaneous casting, although the feats Signature Spell and Extra Spell can allow to increase the number of spell known.
If no spell is prepared on a spell slot affected by this feat, the spell slot can be used to cast spontaneously a known spell.
Special: You can take this feat multiple times. Its effect does not stack (you can't prepare several spells on the same spell slot). Each time tou take this feat, you must select another spell slot.
Another feat prerequisite, and a harsh one since Extra Slot (from Tome & Blood) is the most useless feat ever for a sorcerer.
In fact, I'm confident Gamecat would find even that version overpowered:
Greater Arcane Preparation v3.0[General]
You are a bit learned in the way of wizardry.
Prerequisite: Extra Slot, Scribe Scroll, capacity to cast arcane spells spontaneously, capacity to prepare arcane spells (either with a level of wizard or similar class or with the Arcane Preparation feat).
Benefit: Choose a spell slot of a level no higher than your Intelligence bonus. That spell slot must be one you gained through the Extra Slot feat. You can now use this spell slot to prepare spells from spellbooks just as a wizard does.
You can now read and write spellbooks just like a wizard.
Preparing a spell requires a ritual that lasts 8 hours and draining 500 XP times the spell level. It also requires a successful Spellcraft check (DC 50+twice the spell level). Casting a prepared spell deals 10d10 hit point damage to the caster.
You can also take the Spell Mastery feat just like a wizard.
Spells known (as a sorcerer, bard, or similarly casting class) are considered mastered and can be prepared without spellbook. However, spells mastered with the Spell Mastery feat are not considered known spells for spontaneous casting, although the feats Signature Spell and Extra Spell can allow to increase the number of spell known.
If no spell is prepared on a spell slot affected by this feat, the spell slot can be used to cast spontaneously a known spell.
Special: You can take this feat multiple times. Its effect does not stack (you can't prepare several spells on the same spell slot). Each time tou take this feat, you must select another spell slot.
As for the Prestige Class option... I'm thinking it should get a bit "underpowered" as well. For example, instead of having to have a spell written in a spellbook, it could be just any spell on the class list, even ones the sorcerer or bard hadn't ever heard of. Instead of having to be prepared before hand, it could be done spontaneously, by just using a full-round action to find him. I would call that class "Mage of the Arcane Order" and also give it bonus metamagic feats and ofter nifty abilities. More skill in the class list as well. Probably more balanced this way.