Firstly, I think that people did not like it because of how slow past it was. Personally I loved gen IV . It was my favourite generation after generation III. Back to topic. Secondly, I have read complaints about how the motive of team Galactic was unclear. I agreed with the people at first but when I played Platinum it all made sense (and when I replayed Diamond and read the text...). Thirdly, people also disliked the fact that allot of the good Pokemon where not available after you had beaten the league and that there was lack of fire type, electric type and water type before the League. (I don't know if that is true and if it was just some 7 years old kid who had written a complain about that) Fourthly, the caves in DPP where torture... Too much Zubat's!!! The caves where long, and they had millions of Zubat's. Enough said! Theres are the only reasons I know of why people hated DPP.
In conclusion, I only wrote what I have seen people complain about in the past and I really like the DPP so I don't know why so much people hated it. (I will read other post after this to read other reasons.)
Edit: The end of the post is weird, but I don't know how to write to make it make sense.)