Richie's vocals were always somewhat akin to the guitar style of a shredder - he tends to overdo it, to oversing. Which is why he was PERFECT for backing vocals, when he had to emphasize certain parts ("WAAAAANTEEEEED") for everything they were worth, but falls SONEWHAT flat as a lead vocalist who has to maintain a sense of overall suspense and progression through the whole song, with a huge resolution in the chorus (or whenever). Constantly accentuating parts makes that effect go to waste.

On the other hand, look at the vocal work on something like Thank You For Loving Me. Sure, the track is not really a classic, but the buildup, how Jon gradually works his way up his range, that's understanding how to sing lead.

This sounds harsher than it is - Richie is still a damn fine singer, and technically, he was very much up there, too. But when each were in their prime, Jon just knew how to use his instrument better. I'm not just talking about "how to get a certain sound out", but really, how to apply it to good use.

Also... Richie does a bluesy rasp way more often than Jon (I can think of one of the last EVERYBODYs in Keep The Faith, but else? At least as far as studio stuff is concerned), but I don't think he EVER did those squeals Jon did a lot in the 80s (think Homebound Train or I'll Be There For You).