I truly do not understand why people complain so much about the numerous evolutions that the 4th Gen added.
Wasn't that the really awesome thing in the 2nd Generation? The fact that some Pokemon like Scyther and Pikachu and Slowpoke and Hitmonlee ALL had relevance to the 2nd Gen through new evolutions?
And personally, I really enjoyed that throwback again in Gen IV. Gen III had, what, two ties to any previous generation? Wynaut and Azurill? And those added nothing to Wobbuffet and Marill. I realize Gen III was more of a reboot, but it didn't have that feeling of unity with other Generations because it was so out there.
Gen IV had a large amount of ties to previous Generations due to all of the added evolutions. And frankly, I think those designs are all really awesome. I love Electivire and Gliscor and Togekiss and Lickilicky and Probopass and Porygon-Z and Weavile and Mantyke and Bonsly and Ambipom and Mamoswine and all of them. The unity that Sinnoh had with all of the existing Pokemon was phenomenal.
Sinnoh was also a blast to play through in Diamond and Platinum. The region was huge, there was a LOT to do. I know that my main goals became:
Get my main team to Level 100
Complete my Pokedex
Get as many locations registered in the Global Trade Station as possible
Flesh out the Underground Base
Along with a few smaller goals. Point is, I was able to DO all of those things in Diamond that I wouldn't have been able to do nearly as easily in any of the other games.
Even the plot was more involving than Gen III's. I felt like Generation III's plot was much more passive--you sort of dealt with Team Aqua and Magma at your own pace, which makes no sense. In Gen IV, you had Team Galactic, which was much more ruthless and evil than any previous Crime Team, and you had to climb to the top of Mt. Coronet to stop the universe from being recreated, rather than just prevent a Legendary Pokemon rampage.
After hearing so much good about HGSS, I'm sure that makes this Gen even better. I'd be able to give a better opinion on it if I had already played it, but I haven't yet.
I'm apparently the minority here, but I think the 4th Generation was all-around the best Generation that Pokemon has ever had. Kanto is overdone but is always nostalgic; Johto seemed very weak in GSC; Hoenn felt disconnected from the rest of the world. Sinnoh has unity, originality, many challenges, and a lot to do in a VERY large area.