-no chemicals on my baby's tender bum (like the gels in disposables - some may have allergic reactions, etc...)
-cloth is a TON cheaper, in the long run. I have a 3-yr-old boy, still diapering, and a 7 month old girl. Total diapering costs? Less than $300, ever. (well, plus laundry detergent & water, but I'd still say I'm way below disposable averages). Cost was our main reason.
-theory: cloth helps kids feel wet more than disp, so they are more aware of it & may theoretically toilet-learn easier. (hence the new "feel & learn" diapers!)
-theory: the heat produced in a disp diaper may cause problems for male development & affect fertility later in life.
-there are so many cute options!

-it just feels more natural to me.
-environmental benefits (less waste, less production...)