To give you an example, I'll pick out a few enemies at random from Appendix IV.
Level 7 Cleric. Feats include Toughness, Weapon Focus (Unarmed Strike), and Weapon Focus (Spear). - Toughness sucks, and Weapon Focus is for Fighters. You have better things to spend your feats on.
Level 7 Wizard. Feats include Combat Expertise, Skill Focus (Know Arcana), and Spell Focus (Evocation). - If you need Combat Expertise, something has gone horribly wrong; Evocation sucks; and ... really... skill focus? Not unless it's a prereq.
Level 15 elf. Class progression: Wiz1/Ftr6/ArcaneArcher8. - A level 15 arcane archer is not much of a threat to the level the party's supposed to be at when they meet it.
Level 13 Cleric. Feats include EWP: Bastard Sword, Weapon Focus (Bastard Sword). - Bastard Sword is a trap, and they've wasted two feats on it.
Fighter7/DwarvenDefender3. Feats include Quick Draw. - Why?! (Note that this character avoids the normal DD problem of not being able to deal when characters take a 10-foot step away; the situation where he occurs forces the characters to engage in melee. Otherwise this would be worse than Ftr10).
Monk6/Tattooed Monk10.... I don't think I need to say anything else here.