I'll dispute the part about no ability lacks a type. I looked at the PHB, MM, SRD, Rules of the Game and RC; none of them explicitly state an ability must have a type. But I may have missed something so I would appreciate exact rules quotes.
Okay. First, Monster Manual page 6 (which I just quoted):
A special ability is either extraordinary (Ex), spell-like (sp), or supernatural (Su).
Next, Monster Manual page 315 has the glossary definition of special ability:
Special Abilities: A special ability is either extraordinary (Ex), spell-like (Sp), or supernatural (Su).
It then goes on to describe those three things... and to be clear
that's the definition of Special Ability! It says so right after that page 6 quote.
Next, the Players Handbook on page 180:
Natural abilities are those not otherwise designated as extraordinary, supernatural, or spell-like.
It's a catch all category for all abilities that aren't the first three. Note that the first three types are "special abilities" while natural abilities just covers "abilities." This is clarified further by the Rules of the Game: All About Polymorph article on the subject, which states:
Natural Ability: This term is a catch-all for just about anything a creature can do (or characteristic that it has) that is not extraordinary, spell-like, or supernatural
So, is that explicit enough for you? The very definition of special abilities is that they must all have one of the three types, while natural abilities is the catch all category for any ability that's not special (such as the ability to breathe air or walk... read that rules of the game article if you're unsure about this point).
The ability to be able to use Spells is gained, so Spellcasting is an ability.
Correct, and it can't be natural because, as you say, it's gained. Natural abilities must be inherent to the physical form, not something you learn or gain or that is ever listed as a special ability (see Rules of the Game on Polymorph and the PHB for sources).
But casting a spell is an action; it is usually a standard action but can also be free, swift, immediate, full-round, etc. Now, is making a standard action (to cast a spell) an extraordinary ability? Or could it even be a natural ability? Or is it just an action separate from an ability?
Abilities are indeed separate from actions. You have an ability to do an action. If the action is "I fly 90' as a move action" the ability is "fly 90 (good)" or whatever. If the action is "I trip him and then get a free attack" the ability is "Improved Trip."
The act of casting a spell is an action (usually standard); it is not an ability. Antimagic field does not suppress standard actions. So Antimagic field cannot be used as an evidence that Spellcasting-ability can only be extraordinary.
If the ability was suppressed then you couldn't do the action. But you can do the action (casting the spell). It's just that the action is suppressed, so it isn't useful. Antimagic fields are a rare case where an ability is not suppressed by the action is. A similar case would be an underwater Solar... he still has his fly 90' ability, but he can't fly because flying doesn't do anything here. By comparison, you don't even have Sp abilities to use in an antimagic field.
So where does it leave us? The Spellcasting-ability is just an ability cast spells with standard actions. A wizard can use a standard action to cast a spell whereas a fighter cannot (that is without the Magical Training feat).
I'm really not sure what standard actions have to do with anything. Antimagic fields say nothing about standard actions... why is that relevant?
Special Attack: Spells is not an extraordinary ability. Please see the rules quotes.
That's the only confusing bit at this point, as everything else is really explicit... and it doesn't fit with Arcane Talent at all.
I also searched the term Arcane Talent and I was really confused when I found Complete Mage p.31, which does not seem to have any connection to the issue at hand. Then I found this: Special Quality: Arcane Talent (see e.g. MM5 p.86, p.87, p.184). It is interesting that it is a Special Quality. Each of those monsters also have Special Attack: Spells. What to make of it?
I didn't see anything about Arcane Talent in CM31. It's MMV that has two creatures with Arcane Talent, an Ex ability that just lets you cast spells. The fact that it's a Special Quality is strange... As per the rules on Special Qualities and Attacks, the two are both basically the same (they're special abilities, but attacks are offensive and qualities are not). Spells are of course weird because they're both offensive and passive. However, it creates the bizarre confusion here that Special Qualities are not gained by Polymorph, but Special Attacks are. But note that MM is the primary source on Special Abilities, so it trumps in the case of a direct contradiction (which this would be a case of, since MM says spells are a Special Attack and MMV is saying they're a Special Quality).
JaronK