The True Master.

[Farael]

This it is an interesting thought, that the body might place limitations on the power of the will, even among the Ainur. I am wondering what might cause this

Well, first of all he would have to use a part of his will to mantain his physical shape, as it is not his "natural" state and therefore he has to spend at least some of his will not to go back to his abstract form. Also by having a body you are more susceptible to other kinds of temptations which could corrode your will... even innocent stuff like food and drink can afect it, yet while he was just a creature of will he had no need for such things

, and if it is true, could this be one reason why Sauron might place a measure of his will in something external?

I guess it was his greed for even more power, as somehow the Ring increased his will/power and it also allowed him to control the other rings, which was what he wanted.