So, I guess I have to defend Gen IV's honour by saying that I have actually found Platinum's story to be the most gripping one in any of the main series Pokémon games thus far? Like, it's by no means very complicated or even that great as far as stories in games go, but the way that it was tied together with the actual gameplay itself and with how the natural progress of the game happened is the best in all Pokémon games to date.
Also, I loved Gen IV's music and geographic designs a ton. At places it was pretty forgettable like no Pokémon game had been before (read: almost the entire Western part of Sinnoh sucked), but it still had some incredibly cool locations and routes that I loved more than their counterparts in other regions. The snowy routes leading up to Snowbelle are probably still my favourite locations in the entire game. And the routes surrounding Solaceon were pretty cool as well, to say nothing of the really cool variety that the Frontier Island had.
As far as its Pokémon designs go, it kind of lost some of its edge, I suppose. Stuff like Luxray, the new bugs, and the new starters were kinda boring and oddly round and cartoonish, but we have to remember that it's still the Gen that gave us the likes of Lucario, Hippowdon, Garchomp, and the cool new evolutions like Togekiss or Gliscor.
For some odd reason, aside from Ruby that I've played so much that the timer just stopped recording, Diamond is my most played Pokémon game ever. :P