Humans make up about a third to half of the critters you’ll run into. Except for some of the tougher critters (liches, vampires and daedra), humans are about as nasty as you can get. The reason is because they have pretty much the same abilities as you do (except they’re all “canned” classes) and aren’t afraid to use them.  They also function at about the same level as you (hint: level while you're inside the dungeon and you'll be 1 level higher than the critters you're fighting).  The exception to this is the spell casters. Generally, they don’t start tossing dangerous spells at you until you’re somewhere between 5th and 7th level. Up to that point they prefer to beat on you rather than fry you and if they do cast a spell, it's usually a defensive one like "Shield" or a minor offensive spell like "Silence". Just about all humans carry bows and prefer to shoot at you before you engage in melee.

Thief Class
Warrior Class
Spell Users