Well Xehanort's heartless took the form we fought at the end of KH1 after taking over Riku's body. You see Xehanort's heartless might not really be a heartless at all. When he gave himself to darkness he didn't return as a normal heartless but merely a heart itself. For some reason he retained his memories and personality. The form we see him in the brown cloak at the start of the game expresses that. Unlike most heartless he lacked a body made of darkness. Now he did have a body of some sort as he wrote reports and held up that cloak, but it wasn't strong enough for his goals. So he took over Riku's body and using (Fill in the blank here: Magic, Darkness, any random plot element) he gave the body the appearance he desired.
We know that the other Organization members had normal heartless. So a strong heart might not be the only thing needed to make a special heartless. We don't know exactly why he came out this way. He himself questioned it. But Xehanort's origin might have played a factor in it.
Well Sora wasn't a lowly shadow. He actually was pretty special. He retained his memories and personality as well. He remember Kairi enough to seek her out after becoming a shadow. Something that is odd as when someone becomes a heartless they lose their sense of self. While Sora was special, something about Xehanort must have been even more special for his heartless being so unique.