What a coincidence. I just happened to replay the gen 4 games!
Hmmm. What's the biggest problem with these games? Honestly, there's not too much to talk about, but when it shows, mannnnnnn it's a huge problem.
1.) The level curve in DP is outrageous. I'm talking about BW post game levels of bs. You're expected to be around the late 60s when facing Cynthia, despite the fact the highest level pokemon you encounter are in the mid 50s. Thankfully compared to HG/SS there was no grinding to be had and the improvements in Platinum are greatly appreciated. No lv. 66 Garchomp for me. ThoughtobefairyouhavethatstupidTOGEKISS
2) The HMs. I'm so glad kids today don't have to deal with HMs anyone. Screw them. The are an annoying incessant roadblock that ruins the quality of the games. You need, Surf, Rock Climb, Rock Smash, Defog, and Waterfall for a majority of the the game. 5 moves taken up. Why can't we just delete those HMs on our own? Just have the badge as a requirement and then let us do what we want. It's so annoying to want to explore an area and need 4-5 HMs. Sinnoh is probably the worst case of this. At least in gen 3 there were plenty of water type pokemon that could learn HMs. Because your options are so limited in DP/Pt you'll struggle to find a decent pokemon to use those HMs unless you want to use Bibarel.
3.) All of the pokemon games for gen 4 have pretty bad distribution overall. In the beginning it's not too bad, but then you get to a point in which you lack certain types (e.g. fire and ice types) that are viable. I'm glad in Pt you have better options but it's still extremely limited. Seeing Graveler by the 4th gym is really annoying. In HG/SS it's even worse because in those games some pokemon native to Johto aren't EVEN IN THE REGION. And the influx of Kanto pokemon make it harder to forma well-rounded Johto team.
4.) The speed. The gen 4 games look nice, but sometimes they lag. It happens once in a blue moon and battles are perfectly fine, but the overworld has weird parts in which your character doesn't mess well with the 3d environment due to the anti-aliasing. It's not a gamebreaker, but sometimes noticeable. And gen 4 DP surfing is also awful. Thank god for Pt/HG/SS. I never appreciate the fast surfing speed until replaying diamond.
Overall, fun games. Not too much to complain about.