Problem is finding something where "I can do what the Fighter does only better" isn't an option.
Finding things immune to "magic" is relatively easy. Finding things immune to I-get-a-Strength-boost-and-full-BAB-with-this-spell is harder.
Actually, this is flat out wrong. Casters either get:
1) Conjurations that ignore immunity to magic (black tentacles and solid fog come to mind, but entangle is good too)
2) The ability to beat things up with massive str and either full bab or just not caring (druid or cleric)
On the other hand, know what hoses nonmagical fighters (in core at least)?
1) Being bigger to the point that you can't get inside their reach to hit them (there are quite a few of these)
2) Flying at a level when you can't (not too common, but there are a few)
3) Teleport at will and long range SLAs (any outsider)
4) Beating you in melee (any dragon)
5) Ability to flat out ignore you (spectre)
6) Ability to outmaneuver you (huge air elemental)
7) Ability to hide from you while forcing you to save or lose every round (pixie)
That's just off the top of my head.