Platinum has the wi-fi club, it has a new storyline bit, an entirely new area (although brief, fun while it lasted), it has another new Battle Frontier, there are new move tutors, the scenery and graphics have been very much redone (and for the better), there are plenty of new minor features such as the Battleground and the Resort Area, ... Diamond/Pearl compared to Platinum gained more than Ruby/Sapphire compared to Emerald. :P
Let's not forget new forms, a much larger Sinnoh Dex, and many more things and improvements.
Not disappointed with the game at all.
... In before Prinny complains about a pokemon thread in the wrong place.