A lot of this is just me preferring DPPt visuals. Many of DPPt's interfaces (HP bar in particular) are very similar to Gen 3 (which look great imo). To discern between DP and Platinum, I prefer the way the player looks in DP, as well as the cyllindrical menu buttons as opposed to Platinum's glossy, dome-shaped buttons
On the flip side, I find the HGSS interfaces (menu, status screen, Pokégear) to be clunky and inconvenient to use. Not having the Pokétch is shit too, because you have to actually go to Goldenrod to check happiness, and half the time, the Day-Care man never calls. The HGSS menus are also ugly as all hell.
Certain sprites (Porygon2, Octillery and Sneasel come to mind) have very nice shading, reminiscent of the gen 3 sprites with their intense highlighting and shadows. Platinum completely reverted this change by remove highlight from a lot of sprites, including the ones I just mentioned, and HGSS almost completely removes it from all sprites except the Hoenn Pokémon.
It sucks they don't have the Battle Frontier, but in terms of just having a Battle Tower, I appreciate how every Pokémon is available there, unlike RS where only Hoenn Pokémon were used. And, to be honest, the Gen 4 frontier is absolute shit. The only facility I actually use is the Tower and
sometimes the Hall, since those are the only two where you can actually use items in your Pokémon and thus, use the sets you breed effectively. So I don't mind DP only having the tower.
As much of a bad gameplay decision it was on GF's part, there's something I strangely like about the new evolutions being unobtainable until post-game (and iirc, no trainers use them either). It's almost as if they were supposed to be a surprise to people who went into DP without knowing about them prior. However, DP could have done better by including other Pokémon to straighten out the type variety. But to be honest I don't give a shit about the lack of Fire-type Pokémon because I almost never use them and instead just run Fire-type moves on my Pokémon.
DP also has the Protect glitch, where moves made to have perfect accuracy by weather or ability have a chance to hit through protect proportional to the different between 100 and that move's original accuracy. So, a No Guard Machamp using Dynamic punch has a 50% chance to hit through Protect, which I think is a very cool mechanic. DP also has increased accuracy for Hypnosis, so Hypnosis-Milotic and Gengar are actually viable.
Finally, though an unintentional "feature," voiding is extremely cool (and convenient in some cases), and DP are the only games that you can do that in. There's a weird feeling being in Seabreak path or Hall of Origin in DP, knowing you were technically never supposed to be there.