It's not that Sinnoh's different. Every Pokemon game has its own set of flaws, some more than others. It's just that Sinnoh's flaws were unnecessarily abundant compared to most of the other games. As
Norne pointed out, D/P's issues go beyond pacing/speed; The regional dex was poorly handled, with many Gen IV Pokemon getting the shaft until post-game, and many types being underrepresented to the point where some important Gym Leaders/Elite 4 trainers couldn't even use a proper team that specialized in specific types (I'm looking at you, Volkner and Flint.), many glitches were present, such as the tweaking glitch, etc. Sure, Platinum mostly fixed the issues present in D/P, but that doesn't erase D/P's shortcomings. Usually when I look at the 3rd version to a set of games, such as Crystal and Emerald, I view them as "director's cut" versions. When it comes to DPPt, I see D/P as the beta version and Platinum as the finished product, which isn't a good thing in my eyes.
But long story short, I'm not saying Gen IV was trash and every other Gen is gold. Hell, earlier in the thread, I went on a complete GSC bashing spree over Johto's own set of problems, and I can certainly write up a list of problems from the other generations. As I mentioned before, I have fond memories of Sinnoh just as much as I do with R/B and every other game, and I do feel like the games are unappreciated in many aspects. But there's a reason why Sinnoh is seen as the black sheep of the franchise.
P.S. Just to keep it clear, no one's disagreeing or bashing your opinions. The whole point of this thread is to post opinions that are considered out of the norm, so there are no "wrong" or "right" opinions here. :p