I have lost three different things at WDW and all three were returned to me. Two were completely without hassle, and the third involved quite a bit of hassle.
We lost DS's hat at the MK. He took it off and tossed it over the side of his stroller while we were walking. I noticed it was gone within a few minutes, but when I retraced our steps it was already gone. They didn't have it at Guest Services when I checked there and they recommended that I pay a visit to the main lost and found office at TTC. They asked for a description of the hat and produced it right away!!

Incidentally, while I was there some teenaged girls came in looking for a lost pair of sunglasses. They kept insisting that they be allowed to look through the sunglasses that they had and the CM refused. They were rather put out about it. But I understand why they won't let people look-- first of all, it would be only too easy for someone to spy a pair of sunglasses that they want and say, "Those are mine!" If they require you to provide them with a description first they are less likely to give the wrong pair out.
The next thing I lost was my keys! Again, the culprit was DS. I knew exactly where it had happened, but by the time I got back they were gone. We were leaving the next day and my calls to lost and found were fruitless while we were still there. But when I called to check on them after we got back home the CM said they had them and then mailed them to me free of charge!!
The last time was when we were checking in to OKW. While DH was finalizing the check in arrangements I went to the general store and made a couple of purchases. It's a rather long and convoluted story, so suffice it to say that about 7 calls to the front desk and lost and found resulted in 7 "no we don't have it's" and finally on our last day we went there in person and were standing there when they opened the storage area where OKW keeps lost and found stuff. We saw the bag sitting on the first shelf, directly in front of the door.

The incompetence of those CMs still annoys me.
But all's well that ends well.