An answer to Blindkobold's contentions:
1. There's no problem with tossing two weapon fighters a bone--they deserve one because for the cost of multiple feats, they don't get much at the moment.
Actually, two weapon fighting is extremely effective under the current rules as long as the situation is right. A two weapon fighting rogue (or more often ranger 1/rogue x) can deal out insane amounts of damage with sneak attacks using two weapon fighting.
Similarly, characters with high dexterities and low strengths can gain a lot of benefit from two weapon fighting--particularly if they can gain weapon specialization as well.
In general, all feats and abilities that deal a set amount of damage work very well with two weapon fighting: Divine Might, Divine Vengeance, foe hunter, favored enemy, etc.
Furthermore, the "two weapon" two weapon fighter is not the most advantageous model. Two weapon fighting, etc. plus shield expert and a spiked shield yields better damage than two weapon fighting and a much better armor class.
Two weapon fighting is quite advantageous even under the core rules. Expansion items (some divine feats, the MotW 2wp prestige classes and feats) further expand the advantages available to a 2 weapon fighter.
2. This doesn't exacerbate the advantages gained with rate of attacks because the new attack will rarely hit.
Actually, the second attack will hit quite often. It is only +1 base attack after all; all the other bonusses still add to this. A typical 6th level fighter probably has at least a 16 strength, weapon focus, and a +2 weapon. Consequently, his second attack is at +7. Since the discussion is centering upon my hypothetical combat between a 6th level fighter and a 5th level fighter, we'll leave it at that (in a larger combat, bless spells, bard songs, flanking bonusses, prayer spells, etc would often boost that bonus to +10 or higher). Consequently, the sixth level fighter is likely to hit with that attack quite often.
Giving the high level fighter the option to use an attack to negate his opponents' attacks is certainly going to exacerbate the already huge difference between levels divisible by 5 and the levels after that. I don't think D&D needs any more power plateaus than it already has.
3. It's not superior to the 2e version--you lose any parry attempts you don't use at the end of your round.
Consider the following situation: your typical 6th level fighter (as described before) is up against a group of one fourth level fighter and four second level fighters. The 6th level fighter has a very good armor class--the 4th level fighter needs to roll a 14 to hit him; the second level fighters need to roll a 17 or higher.
On about three out of every four rounds the 6th level fighter can expect to be hit at least once. (The odds of all of them missing are .8^4*.65 = about .25)
Since there are five possible attacks none of which have a particularly high probability of success, it will be hard to predict which attack will be the one to hit. So, if you had to choose which attack to parry before the attack roll was made, the parry feat wouldn't make a big difference to the likelihood of being damaged (although parrying the fighter 4th level fighter who probably has weapon specialization might make a significant difference in the amount of damage taken).
On the other hand, this feat allows the character to negate whichever one hits which will mean that most of the time, he will take no damage (as opposed to 3/4 rounds).
4. D&D currently has a lot of abilities which allow characters to avoid damage without boosting AC.
You're right about Defensive Roll. I haven't looked closely at Song and Silence so I can't comment on the Outlaw of the Crimson Road. . . . The big thing is that most of these abilities function only in unusual circumstances (a blow that would otherwise have killed the character in the case of Defensive Roll, for instance). That's very different from a second rank feat that allows you to do this to the first attack that hits on every round (at a cost).
5. This feat only grants the ability at a huge cost--the loss of an attack.
While I don't dispute that the loss of an attack is significant, I don't think it's as huge a cost as you describe. For two weapon fighting characters, it doesn't cost much. For two weapon fighters with shields (and shield expert), it costs even less. Any time a character has more attacks than an enemy, it's a good deal. It's also a good deal when a lot of the enemies attacks are likely to miss and it's an insanely good deal when fighting a creature like a troll which has very deadly attacks (hint, if one of its claws hits with a low number, you can negate it and avoid the rend. If a claw hits with a high number, you can not bother trying and just attempt to prevent the next claw (if it hits you) from hitting.
6. Odds are always in favor of the attacker.
Not so. In the typical one tough fighter vs. tons of scrubs fight, the scrubs have trouble hitting the fighter's AC. A mid level fighter stands a very good chance of negating the few hits that occur due to natural 20's. (If your attack bonus is +12 or +14 (which is easy to get at 7th to 8th level), it's easy to beat the 24 that scrub Q ended up with rolling a 20. And on opposed rolls, 20's don't automatically succeed. . . .) This feat would make the fighter practically invulnerable to gangs of scrubs in melee.
Good catch about the missile fire though.
7. Like all feats, it's a boon in some situations and useless in others.
That doesn't make it a good feat though. A feat which turned every arrow into an arrow of death against dragons or that prevented any wounds the character dealt to his foes from regenerating would be a boon in some situations but useless in most situations. That doesn't make either of those abilities balanced as feats.
8. Feats let you do what you already do better--there's no logical problem.
OK, you're right here.
9. A 2 wp fighting shield expert could be very effective with this feat.
That's part of what I have a problem with. In 2e, shields were worthless because the monsters hit most of the time anyways. On the other hand, two weapon fighting or bladesong gave an extra attack that could be used for a parry (which usually negated an attack). So the best way to construct a defensive fighter in 2e was to either be an elf and use a longsword in one hand and nothing in the other or play a human and fight with two swords.
IMO, the best way to construct a defensive fighter should be to use a one handed weapon and a shield. You shouldn't need to get two weapon fighting in order to make your shield effective defensively. This feat returns to the days of 2e where the most effective defensive fighter is the fighter with two weapons.
10. Different isn't always bad.
Actually, when it comes to mechanics, I think that different usually IS bad. There's a reason that the knockdown feat and the wolf's improved trip ability use the mechanics for the trip action. The fewer mechanics there are for the same thing, the easier it is to remember how to deal with situations in combat. (I know that Stand Still and Large And In Charge do the same thing with different mechanisms. I don't much like that either, but I understand that it's necessary in order to maximize the usefulness of the feats for the sizes of creatures that will typically use them).
Other miscellaneous problems: As written, the feat doesn't require it to be used with a melee weapon. It'd be ridiculous, but according to the letter of the feat, a character with a bow and multiple attacks (or just the rapid shot feat) could make a full attack and save one attack to negate any charge attacks that managed to hit him.
