hard - fast - mostly on the shoulders - with control
I was working with the kids class a while back on this idea of throwing osotogari for ippon (onto a crashpad).
We talked about the concept of a perfect throw being one that is strong, fast, lands uke mostly on his back of his shoulders, and shows great control.
Then we talked about moderating our desire to throw perfect throws in randori because they are too punishing and hurtful for uke. With your friends in friendly randori, you should emphasize control and don't worry too much about hard and fast. In shiai or street, hard and fast are okay but you always have to maintain control. Control is the most important facet of ippon.
[photo courtesy of Misha Penkof]