Warriors/Fighters are brute force, little more. Beyond healing themselves with potions, they are not very magically equipped - as in, casting spells. The best source of thier power are cirtainly the high Base Attack Bonus, Bonus Feats (Either things like Rage or Weapon Specilisation and so on), High Hit Points and heavy armor.
In NwN, most non-spellcasters are fighters. For non-PC classes like Undead, these can be considered hybrids of fighters, and some do have the full attack bonus (see the first Building a Better Monster file for that information).
Monsters normally uses thier attack bonus as thier main weapon. High BAB or good special abilities or damage will be a must, at least to be able to hit the enemy (much more important at higher levels with 30+ AC with spells!) or at least threaten them. As they cannot dispel, cast spells or do anything really fancy like hiding, sneak attacking and so on, they should be able to hit the enemy and survive being hit themselves! Else, they are mear cannon fodder (and that might be the point...)
For the best results (or mainly, any results), they need support regularily, to either be healed or helped by spells. They can, though, perform a wide array of actions. They will usually be the ones patrolling the complex, guarding an area or doing weapon crafting, practice, and so on in the art of battling. They may use a ranged weapon - which makes them twice as hard, or should do if used correctly, at least.
The reason for support is mainly thier major vunrability to spells, saving throw checks and anything that stops thier blows - even healing, byond thier own potions, is impossible. Simply put, they fight, and can't do much else.
Thier tough armour also stops many blows, if they are a heavy fighter. A lesser version - rogues and barbarians, perfere less armour. Rogues Dex bonus is so great, that probably matters little in comparison sometimes (especially with no magical enchantments).
Mostly, as toughness goes up, damage of weapons, and AC does. Hit points are almost the most important thing as they don't have magic. Monsters are different - and abilities depend upon thier skills. Some are magical, some have abilities and so on. Feats are an integral part of it as well. I love giving giants knockdown and seeing them knockdown everyone, if thier attack bonus is high enough to get past the disciplin. Monk feats are automatically given, and work well - very annoying, as they are basically power class, even when given the little meager weaponry that the basic "grunt fighter" gets.
Main thing is to get the balance between being able to hit the enemy, and not knocking down equal levels in one hit, unless you want them to. They are usually underrated, and spicing them up with different abilites, classes, waepons and so on really helps to make them special. Also, having different levels of the same creature makes a lot of variety - think instead of all level 5 trolls, a few are level 3, some level 6 or 7. Maybe a garrison has some good warriors, some bad ones, some archers, some knights, whatever. Cirtainly, if a cave/stronghold/house contains no mages, clerics or rogues, please please please make the fighters have some variety.
Thinking on the lines of vareity, different types of weapons and different levels and feats (especially feats of any type) help to really get a dungeon on the right lines. Having 1 sort of fighter, from Goblin to Demon of the Apocolipse, the PC's may find it very tedious (I know I would) as, for starters, it will be too easy or too hard for the PC (or party) and makes it very boring. It is not hard to make a copy of an exsisting blueprint, alter a few feats, HP, AC and so forth and add it to exsisting encounters.