I, for one, miss the time period of Gen 4 (2006-2010). Only a few years ago, but still some good times. It was a very good part of my life: Red Sox winning the World Series, Celtics winning a Championship, Patriots going >18-1. However off-topic this is, I just wanted to share it.
Wifi Battling became my favorite thing to do. I'd assemble my team and fight randoms from an internet chatroom. There was no better feeling than defeating someone online!!! I also loved the Battle Tower, though I prefer the Gen 3 Battle Frontier (Thank you Platinum for bringing the Frontier back!). I almost forgot, the GTS was awesome! It also gave me my competitive team.
I was so excited for Platinum that I bought it in Japanese off EBay (Well worth it). I loved the revived Day/Night System, the different soundtracks to accomodate for Day and Night, and the upgraded overworld graphics.
Gen 4 had some of my favorite Pokemon overall (Weavile, Azelf, Porygon-Z)and gave some much-needed evolutions (Mismagius, Probopass, PoryZ again, Gliscor) which gave some usage to otherwise forgettable Pokemon.
There are some aspects to Gen 4 that I don't miss, but overall I liked it very much. There's no bad generation for Pokemon.