4 years ago, I got Pokémon Diamond.
I already played Pokémon since Blue came out in Europe.
However, in Diamond, I was able to meet new Pokémon, explore a totally new region, and I had a lot of surprises.
It already felt like a totally new game!
This has gone away in Platina (and Pearl), since I already was familiar with Shinou.
Heart Gold and Soul Silver also didn’t give me the fresh feeling, since I’m also known in Jouto and Kantoo (GSC).

And now, 4 years after Diamond/Pearl, I got Black.
Here are only new Pokémon, a whole new Isshu Region, and lots of new surprises.
I never had as much fun as 4 years ago!
The game itself feels really fresh, and Isshu makes good use of really natural areas, and really urban areas.
2 rivals show up really often, and so is N, which was later changed from Pokémon Trainer N, to Plasma Gang N.
The Global Link and High Link features are also really good points of the game, but I didn’t see enough of this to rate this.

The thing I don’t like, is Dream World.
The website contains kiddy minigames, and after connecting once, I never had the ability to connect again.