I love Diamond and Pearl. I would consider the wombo combo of DPPt and HGSS to be the best overall generation of Pokemon games.
+A great array of starters. Not only were all their designs amazing, but they had really interesting type combinations as well. Yeah we got Fire/Fighting again, but surely this is for the last time!
+Brilliant overworld. It was very intricately designed and varied. This is where they really started to get ambitious with the different locals you travel to. Snowy peaks, dark caves, volcanoes, modern cities, sludgy swamps, rustic towns, flowery meadows... the game had it all! Not to mention the underground was a fun distraction with good rewards.
+Return of the day/night cycle. This was completely taken out in Gen III despite being one of my favorite features in Gen II. Instead of just having morning, day, and night lighting, there are many more options making each area look a bit new everytime you pop on the the game. Pokemon appearing at certain times of the day also returned, as did daily events. Don't forget to catch a Driftloon this Friday!
+The physical/special split is one of the most important aspects they added. It completely opened up what they could do with how Pokemon functioned in battle. It forced players to think far beyond type combinations and diversified roles of Pokemon.
+WI FI I CAN TRADE AND BATTLE POKEMON ONLINE MY GOD. We take it for granted now, but at one point trading Pokemon over the internet was a radical concept.
+ Tying in with the Wi Fi, catching them all was now a realistic process in solo play. The game also introduced so many different ways to catch and evolve Pokemon. One of my favorite aspects is grabbing as many monsters as possible, so this game really catered to that.
+Solid post game material. There were a lot of legendaries to capture (perhaps too many) and things like the Battle Frontier let you hone your team.
I do have to say that a lot of the new Pokemon aren't that great. Garchomp, Rotom, Dialga, and Spiritomb remain some of my overall favorites, but so many are just there. A lot of people complain about the new evolutions of old Pokemon, but I found many of them to be creative. A lot are kind of locked until later in the game though. Despite this, I think they diversified the Pokedex enough so it felt like you were always finding something interesting.
And personally, I never found the games slow. Perhaps if I was to play right now I would notice it, but at the time I didn't notice it.