In gameplay, there is a number of differences. I'll tell you, I have a great memory of Platinum. xP
Um... I know there's more, and I know I've gotten a little detailed. :P
Soz about that. I just miss that game so much. :')
Can't see myself restarting it any time soon though. Hm. ;(
Hope I helped. :)
In Diamond, you find Dialga, in Pearl, you find Palkia, and in Platinum you find Giarantina ( or however you spell it).