Basically, the idea behind fighter's cunning is that they use their knowledge of opponent tactics to hide their real attack. They sacrifice their first attack, which becomes a 'cover' for the real attack that will strike at an opponent's weakspot, and therefore do a ton of damage should it hit. It plays off the idea that fighters over time learn how various people are likely to respond to various apparent threats and will start using that against them over time. It is essentially my attempt to give the D&D high-end fighter some of the advantages of the d20 Modern 10th level Soldier (the closest comparable d20 Modern base class/AdC/PrC to the D&D fighter), who can sacrifice an action point to auto-confirm crits. However, the Soldier has only +3/4 BAB, while the Fighter gets +1, and D&D 3.5 lacks action points, except in specific campaign settings, so I also felt it a good idea to limit it to the 2nd or later attack and put in a skill check requirement (though I'd also be willing to use Knowledge (tactics) here, as another choice) to prevent it from being an automatic thing that will always be used if a player has put a heavy investment into boosting the threat range of their weapons. (having doing crits half the time on an attack can be pretty brutal, especially if you can do so more than once a round)
I'd consider Tumble for a substitution class, or a variant base class, where they drop a couple of skills in return for Balance and Tumble (so that they essentially choose a lighter-armored route focusing on Dex or a heavier armor route focusing on Str or Con) because giving those skills means taking over several of the combat advantages of the lighter-armor classes, as far as I can tell, so giving them the skills plus better armor can be quite powerful, especially once the armor vs. spells starts kicking in to offset their generally low Dex scores. (+5 armor and a +5 shield would mean +10 touch AC!!!, rendering their touch ACs easily in the range of a monk's, though the monk gets some of his bonus for free, and would provide a VERY good incentive to go sword n' board mechanically) However, I'd have to playtest the new modified version before I could say for sure whether or not it would be appropriate.
Some of the other houseruled skills (Profession being one of them) are simply because I find the idea of fighters not being good at doing things other than simply thwack things less than intelligent, and wasn't at all related to improving combat ability, but just what I see as bad flaws about their class design. (I can see Barbarians not getting it in most campaign worlds, but considering that fighters are in some sense a catch-all class, it just seems counter-intuitive, and the same goes for spot/listen and wizards vs. fighters)
Were I to have knowledge (tactics), I'd basically allow PCs to use it to get hints about their enemies, once they have heard something about how the enemy fights, or for use in gaining bonuses vs. enemies in combat. Nothing major, perhaps on the order of a +1 or +2 to attack rolls or somesuch in combat, but enough to be worth some effort, especially as those bonuses couldn't be dispelled. I'd also be killing the separation of monster types as knowledge categories but compressing them into a catch-all 'monster lore' category, which would probably be mostly cross-class.