1. Paralysis should have, more than the average spell, a good chance of failing to work.
2. There should be a charging time for a weapon or spell usage for paralysis.
3. It should be really really expensive.
4. And this is my preffered idea. As a default, you cannot paralyse someone unless the effect has already worn off. Also, there would be a set time (as part of the spell and shorter of these times cost more magicka) where the effect is still technically on (so you can't paralyse them again) but they have freedom of movement, to allow them to continue battle without being instantly paralysed again.
Thoughts?