To expand on this point, the 3e Warrior class was virtually identical the Fighter class, save that it had no bonus Feats.  Yet, when calculating the Challenge Rating of a monster that had Warrior levels instead of Fighter levels, 2 Warrior levels were equal to 1 Fighter level (to a point).
I remember my players crying in dismay when they found out why a recurring Ogre Mage NPC was able to keep up with them in combat every time they ran into him- because I kept giving him Warrior levels!
"Surely by now, he should become a Fighter?  How can he stay a mere Warrior?"
And my personal favorite, 
"So if I gave up my bonus Feats, could I double my level?" from the party's Fighter.  : )