I feel like with Gen 5 and 6 it gets unjustly hated for that one reason which in 4's case seems to be the slow battles. Which I don't really mind once your playing that Gen for a few hours you get used to it.
Gen 4 introduced the much needed Special and Physical split plus all the wifi stuff it introduced. It was probably the Gen that was one of the biggest game changers.
Personally my big problem with G4 is the terrible Sinnoh Dex (yes, I know it got fixed in Platinum. I never bought Platinum. I played Pearl and skipped Platinum because I didn't like Pearl).
Fire-types: Chimchar, Ponyta. In a region with a Fire-type E4!
Electric-types: Shinx, Pachirisu, Pikachu. Final gym leader is Electric!
The fact that Electivire, Glaceon, Leafeon, Magmortar, Porygon-Z, Tangrowth, Magnezone, Rotom, Lickilicky, Rhyperior, Togekiss, Yanmega, Mamoswine, Frosslass, Gliscor, Gallade, and Probopass were all locked behind beating the Elite 4 (again I know, Platinum fixed it) was really awful to me. That's like a quarter of the new dex locked behind the E4.