I have always wondered why so many people dislike the Sinnoh games. Personally, I love them, and while they are not perfect, they are definitely great games. But as for why other people dislike them, there might be many reasons. Here are a few minor ones I can think about, some which I have seen people complain about while others are things that I can imagine people would dislike:
First off, if we compare D/P/P to previous games in the series:
- D/P/P does in general have longer save times than any other games in the series.
- Important trainers have dialogue in the battles. A very minor thing, but I have seen people complain about it.
- The contests changed from R/S/E, and the Pokeblocks were replaced with Poffins.
- The bag had unlimited storage, which meant you no longer could deposit items to the PC, but had to scroll through a lot if you had many items.
- The games had very many legendaries. I have seen this complaint once (but then it was about R/S/E), although some people might say the same about D/P/P.
- The story might have been a bit strange and hard to understand at times (like what the actual motives of Team Galactic were). But this was somewhat fixed in Platinum and a very strange complaint seeing as the stories in the Pokemon games never have been anything special anyway.
- The Poketch only had one button in D/P, which could make it annoying to scroll through all of the apps just to find the one you were looking for. Platinum fixed this by adding a second button.
- The surfing speed in Diamond & Pearl was very slow, but this was fixed in Platinum, where it was faster.
- It wasn’t possible to ride the bike through gates in D/P, something that had been possible in most older games. But this was also fixed in Platinum.
- No Battle Froniter. People who had previously had fun with this in Emerald might have been disappointed when it didn’t reappear in D/P. But it was back in Platinum.
- Being unable to rebattle Gym Leaders. After playing Emerald, I guess a lot of players expected this to return in D/P. But it was brought back in Platinum.
- For the PAL Platinum only: The slot-playing in the Game Corner was removed. It might have made some people angry.
One thing mentioned in this thread is the slowness of the games. The 4th gen games run in 30 FPS compared to the 3rd gen games which run in 60 FPS. This makes them slower to play (I guess that is the main reason, although there might be other factors as well). But, according to Bulbapedia, Platinum is a bit faster than D/P despite having the same framerate (I have personally not noticed any noticeable differences, but I guess others have).
Another thing mentioned in this thread is the Pokedex issues. Certain types are definitely underrepresented, especially Fire-types in D/P where the only choices were Infernape and Rapidash. Not being able to use all of the new Pokemon before the E4 is another thing that players might have found annoying. But this was another thing that was mostly fixed in Platinum.
If we instead compare D/P/P to the newer games (HG/SS & B/W+2), this could be for the people who played these before D/P/P or went back to the Sinnoh games after playing the newer games, here are some things people might dislike:
- Not very good use of the Touch Screen. Compared to the newer games, D/P/P didn’t use the Touch Screen much, mostly only for scrolling in the items menu and for the Poketch. And for battles, but all of the DS games have done that.
- They had no Pokemon following you (outside of Amity Square). I will never be able to understand why, but a majority of players like this feature in HG/SS. For those who liked this feature, not having it in D/P/P might have been a disappointment for those who played HG/SS before D/P/P and those who returned to D/P/P after playing HG/SS.
- D/P/P lacked something to connect to outside of the games, HG/SS has the Pokewalker and B/W/B2/W2 has the Dream World.
Even with all of this, the main complaint I have seen is that people don’t like the designs of the 4th generation Pokemon. What Pokemon you like and dislike is of course something very subjective, there is nothing right or wrong with liking or disliking certain Pokemon. But I have never seen anyone go into a deeper explanation about why they dislike the Sinnoh Pokemon.