I'm not a "long-time pokemon fan". Indeed, I've only started playing since Generation 4. Pokemon Diamond was my first ever pokemon game. That was, in my fading memory, when I was about 8. Though it was very fun at the time as well, after experiencing competitive battling and getting to decent levels of competence within it, I came to find that DPPt represented much more than just a childhood game. After learning to only play the best pokemon and strategies in competitive Gen 4 - 6, I found extraordinarily nostalgic that the pokemon I loved (and still do love) back then still do excellently. Though this is a personal experience, I'm sure other Gen 4 players would still appreciate this as well.
Anyway, I'd just like to list some things that made DPPt one of the best games I had ever played in my lifetime. Of course, I've played HGSS, BW, BW2 and XY (and even tried my hand at GSC), but I simply found they don't hold the same meaning for me as Pokemon Diamond. I only viewed them as something to introduce me to a new generation so I could get into the new metagame quicker on the competitive scene. Even now, DPPt is the only generation I can play just for the sake of playing; nothing else.
Starters. I personally believe Gen 4 holds some of the best starters in the history of the game, both competitively and design-wise (and ingame performance, of course). I originally started with a Piplup when I first began playing, but as time (and the number of playthroughs) passed, I found myself trying out Chimchar for its sweeping capabilities, and finally Turtwig (which I found, though I had looked down on it at first as weak and slow, back then when I believed the only good pokemon were ones that could 1HKO most opponents), which makes a great tank with Synthesis. Especially Empoleon and Torterra had great designs, that while nothing awe-inspiring like some legendaries, still grow on you as time passes.
The Starly Line. God, I can't express how nostalgic this line of pokemon is; I can confidently say, that for me, it represents my Pokemon-related childhood. Starly was (and still is) a great representation of potential power and cuteness simultaneously. Of course, Staravia isn't that great (though he was called something else; I originally played the Japanese versions), and Staraptor's looks are just extremely cool. I still remember my first time fighting the Veilstone Gym with Staravia and almost getting 6-0'ed by her Lucario.
BGM Music. Whether it be the Eurostyle pop of the Game Center, the catchiness of Route 225, or the Champion Battle, DPPt's music had always had a special place in my heart. Of course, BW has some great music (especially the ending theme), but I guess that's what nostalgia does for you, I suppose. Especially the Cynthia battle BGM. How cooler can you get?
Dialga. Though not in the same sentimental class of the other elements, I still remember it as the first pokemon I ever saw - on the front cover. Combined with the epic map design of the battle location (I forgot its english name; I still remember it as the やりのはしら. Still incredibly Japanese, I know.
And the best for the last... Alakazam. I don't know where to start with this pokemon. I originally received it with the in-game trade (fondly named ケーケー, or "Kay-Kay" in direct translation), and it quickly grew to be my all-time favorite pokemon. Add that to the fact that the nickname was, in fact, the name of my best childhood friend who died of a car accident several months before I first bought the game, and you get one hell of a sentimental image. It was the first (and perhaps only) pokemon, ingame, that could 1HKO a pokemon only several levels below with a neutral attack. It was my first Level 100, when my other pokemon were just nearing 50. I used to solo most Gyms with it; with a moveset of Psychic, Energy Ball, and Shadow Ball, I even believed it was the only pokemon that ever saw stage-time in the E4 and the Champion Battle.
So there you go, a one man's relieving of DP nostalgia. Sure, I continue to look forward to new generations, but I find that Gen 4's the only game that I can return to my childhood days, for a while. Share some of your nostalgic memories here. Who knows; we may be able to relate.