Truly enough, the Pokemon anime franchise has a lot of uphills and downhills, starting with the Original series with the Johto season to the Advance that considered average, Pokemon Diamond and Pearl or for short DP is definitely one big uphill before the Black and White downhill, and DP is a good one especially because it's originality and maturity. Ash (or Satoshi in the Jap ver.) starts a brand new journey in the Sinnoh region who - as the other regions - include new generation species of Pokemon and Pokemon League of its own with 8 gyms scattered around the region, there he meets Dawn, anew trainer who aims to be a Pokemon Coordinator and his old friend Brock, he also meets a new rival called Paul (Shinji in the jap ver.) and the rest is pretty known... so what is new and different that make DP maybe the best series in the anime franchise?
Well, let's start with Dawn, that though she came to replace May, she is much more girlish and more ambitious to be coordinator from the start of her journey, especially since her mother was top coordinator, unlike Misty and more unlike May who both had a bit of tomboy personality Dawn from the start showing much more girlish side than Misty and May together, so though she wants to become a top coordinator she has some issues of her own. As a male though its can be annoying its actually makes her a really interesting character that adding interesting and important side to DP. the main reason for that - unlike the Advance series that May's coordinator journey feels like "side story" (as in the game the contests are a side game) Dawn's contests journey is much more involves in the story and feels canon, Ash is much more involves with Dawn journey, especially since Dawn knows much less than May about Pokemon that Ash influence Dawn in more than several ways. and even the opposite - Dawn influence Ash's journey, both with character development and in Pokemon training.
And that only Dawn little part in the anime, much larger part of the anime is about Paul, who from that start becomes reason for Ash to get stronger - not to beat the league but to beat Paul, their rivalry seems not just important but making the Pokemon League story much more minor and their rivalry the main story. it might be needs more expanding to understand but the main reason for their rivalry is training-point-of-view, which makes Ash more determined to prove himself to be worthy trainer by beating Paul - an actual goal that makes sense considering his journey to become "Pokemon Master", and so their rivalry taking huge place in DP as their encounters in DP are making the waypoints along the series.
Except for those points there are also Team Galactic, who in some ways might be the best villain team adaption in the whole Pokemon anime merchandise, their story involving Ash the gang are so mature at times you forget it's the same Pokemon anime. Some of the story have some corny points that brings you back to the Pokemon you know but it's not making the story of Team Galactic less interesting.
And that is not only what's new on DP, compared to the other series as Ash's Pokemon league journey might not changed much but feels much more sequel to the Advance series - if in the Advance series Ash had much more confidence and some Pokemon battle strategy, in DP this is entirely different case - strategy is not anymore "Get out of a close fight״ but literally strategy: calculating damage and moves, creating situations for winning and more than once using other advantages than types. and as I said earlier, Ash is influenced a lot from Dawn contest battling styles to use in battles, adding a lot to his battle style and dedicating himself much more to train his Pokemon than battling them only in gym battles.
You might say - so what? DP containing 4 full seasons, it sure have a lot of boring and predicted story episodes that filling the gaps, while it might true but less than you think, the above points rules a lot on the series than you think, either it's Ash's VS Paul episodes, contests episodes, Galactic Episodes, gym battles and league episodes, it seems that the writers created whole new series fixing not only Johto episodes but created whole new style.
But... and that's not a big but, there's some minor episodes who really can be boring and some even worse than the Johto episodes, for those come much rare and not often that you can count them in one hand per season, and some forced changes that the writers brought to make some team change or other story, and more than that,showing a lot of Pokemon we saw already, but again, it's really not that bad to make the DP series bad and even when they come they are easily forgotten (though some may really bothered me at time watching them, such as older Pokemon episodes).
Story - 9.5
Character
As I said earlier, Ash written better than ever, he may have sometimes his regular reckless and corny character, but in DP he's not just more confidence and calculated but more mature, if I might say Ash is not just designed in DP older than 10 years old but also acts more mature than the Advance series. And more than that his point of view as a Pokemon trainer is more essential in DP in his rivalry with Paul - his love for Pokemon, but not just love but his belief that once the trainer believes in his Pokemon they can do better than their usual powers, the interesting part is that Ash combines it with his calculating mind - once he thinks on strategy his straight solution is definitely believe in his Pokemon and no more, and this solid fact is what deepen the rivalry with Paul.
Paul on the other hand, not only doesn't believe in his Pokemon but have more strict approach not only in his Pokemon training but also in catching - if the Pokedex says the Pokemon isn't high level and doesn't have powerful moves - he won't use it but release it. More than that, if while training the Pokemon he doesn't seems to be levelling up well - he will release it, this main approach is what make most of his rivalry with as I explained Ash's training point-of-view.
What interesting is, that in the games Paul approaching isn't wrong, in the contrast - usually you will want to find more high level Pokemon with good stats than raise a weak Pokemon, but since we know Ash already from previous seasons that Ash' approach is good and useful you won't only cheer for Ash but also be interested in Paul's development through the series to see their ideals clashing.
Dawn on the other hand makes much more rivals than Ash (though Ash meets Barry later, Dawn still have much more), starting with Zoey, to her Childhood friend and more rivals that keeps coming along the series. but that's not the only thing about Dawn, though she sure in her own way she still have a lot of downhills along the way, even some you won't even except. And since she's written more girlish than other heroins in the Pokemon series she wants you to cry with her, be excited with her, be interest in her journey. Though some of those seems forced Dawn still makes a really good and interesting character, it can be noticed that when May make her comeback in the series you can really see how much May is flatten character than Dawn's deep character, sure she's not the perfect character as I said but she still have much more to offer than May.
Brock on the other hand feels much more necessary in this series than in the Advance series and its seems the writers have much more leads to his character along the journey, leading to Brock's own solution at the end of the series.
Barry is a new rivalry character to Ash presented at some point in the series, and though he is much unlike Paul he can steal the show at times.
Team Rocket at the other hand, have interesting change through the series, in the start of DP it may seems like they were forced to be in the episodes only to justify their existence, but as the series continue they seems to change to their old-indigo-arc character, not only in their role but even to be funny and original than annoying. Not only that but even Jessie has her own rule in the series performing in the contests to makes things little more interesting, both in her appearance and her contest battle style.
There are a lot more to say about the character in DP series and that's says a lot about DP - unlike the previous seasons there is a lot of Character development in DP, not only between Ash and Paul but also in Dawn, her rivals and Brock, the gym leaders is introduced and well written, but I won't extend more than that since the general idea cleared already.
Character - 9
Art - 7
Though the Art doesn't change much or even a all through the series, it's still better than the Advance series and is more permanent and can really be good at times, especially in some main battles. though one main problem is the backgrounds the may doesn't seem anymore like child play but still seems as drawn bad.
Sound - 8
For some reason, DP have a better OST than the others Pokemon series, and it's shown well in battles, while talking, walking, winning or losing, it may still sound kinda old and mechanic but way better than the Advance series.
Enjoyment/Overall - 9/9
DP feels as sequel to Advance than any other Pokemon series, Ash's journey may be long but still fun to watch and wait for more Ash VS Paul than gym battles, their rivalry is really much more central than Ash ambition to be a Pokemon Master or win the league, and that's make one really good memorable Pokemon series. too bad to see something equal to DP only after the BW in XY, DP is truly one of the best series in the Pokemon franchise.