Im all in favour, an idea mentioned above, for a new effect called 'ground'. available for poisons, spells, enchantments.
When cast on NPC or player it will dispel levitation and prevent levitation for duration of effect. You'd have to think to levitate. 'if i get too high that guard will fire a ground arrow and ill die cause Ive been ignoring acrobatics.'
Im also in favour of guards having bows and the AI to deal with an airborne enemy. Also flying enemies, spellcasters and whatnot.
I do think a simple wolf or similar should be stumped by levitation. In that case, they should flee, not hang around to get picked off.
In summary, levitation is a wonderful game mechanic that opens up the world so much, allows freedom of travel and viewpoint and adds a whole new dimension to combat. What is not to like?
Play warrior and dont touch it if you dont want, but Im a mage. I get hit bad by a melee warrior, but I have the illusion, destruction (and as argued in this thread, alteration) to deal.
edit: One of the things that increased immersion for me in Morrowind were cliffracers. I know, they were overly abundant. But, i had to look at the sky. it was 3-dimensional.
I sorely missed airborne enemies in oblivion, even if i couldnt fly. It made the game more.. flat.