Generation IV was a decent generation, but because I liked it doesn't mean I'm not aware of its shortcomings.
In Diamond and Pearl, I really didn't like the rival, Barry. He was so speedy, it was annoying... but they 'fixed' him a little in Platinum, where while he still couldn't stand still for a second, he was bearable; in particular he seemed like a much better character when he stated that he was the one who was planning on running into the grass and that Rowan shouldn't penalise you because of his mistakes.
The other thing that bothered me was the fact that a lot of the Pokemon from Sinnoh were evolutions of Pokemon from previous Generations, and with the lack of Pokemon of certain types (point and case, Fire-types), it kind of dulled down the experience for me, a little. While I don't mind evolutions of past Pokemon, primarily because I'm still holding out hope that they'll round off Eevee with a Normal-type and Ground-type evolution, it was the number of past Pokemon they decided to evolve that bothered me. It was almost as if they were trying to get all the evolutions that they wanted of past Pokemon so that they could be 100% original-ish with Generation V, looking back at it.
One thing that I've heard people mention before (not necessarily from here) is that there were too many Legendary Pokemon. I only tend to agree because most of the Legendary Pokemon end up being Event Only Pokemon, such as Shaymin, Darkrai and Manaphy, to name a few. I'm fine with Generation IV on that score, though; partly because it doesn't do the trick that Generation V does with one of its normal Pokemon. Could you imagine if you could only get Riolu if you had a Mew from the previous Generation in your party?
I'll have to admit, I was also more interested in the Generation III contests; Generation IV's had too much going on in them, and part way through, I kinda got fed up of them. Though I was happy that they left them all in one place, unlike in Ruby and Sapphire, though it did make it feel like an additional journey, in that respect...
But as for the rest of the game, I'm pretty happy with Generation IV. Like I said, not everything is going to be perfect and there'll always be something that someone doesn't like, but so long as you get an enjoyable experience out of it, that doesn't really matter, does it?