fallout did this ok, radiation resistance was not magic, glowing ghouls had an awsome explosion effect that was not magic etc. i think bethesda needs to differentiate between spells and effects.
firstly, armour could give of effects such as penalties to agility, warmth from blizzards, make the player more intimidating. the problem is that if everything was magic then we cant enchant this armour.
another problem with a magic only system is that we dont want mundane animals using magic, we dont want adrenalin being used as a spell (you might, but i dont see it very logical, i think adrenalin should function as adrenalin)
bonuses from having normal effects different from magic effects
-armour being enchanted even though it already has some effects of its own (understandibly, you shouldent be able to do backflips in heavy armour, daedric armour would make you very scary, but would only give you a slight resistance to the cold-unlike fur)
- there could be unique spells that do not require magika but do require energy/fatigue/action points/health (not saying the player gets these, but it would be cool to see a creature completely change strategy)
- effects that could set off on their own (adrenalin, perhaps some form of emergency ability when near death etc)
-resistance to dispell for effects
- commands (go here, charge, be cautious, retreat etc)
- group buffs/enemy debuffs in a realistic way (warcries, terrifying/inspiring presence ,taunts etc)