Let me throw my two cents on the matter here.
First off, Diamond and Pearl. What people DO like the Sinnoh games for is the fact that they introduced Wi-Fi battling, and some of the 3D aspects that the series was introduced with. And of course the physical/special slit - but that's about where the significances end. The games were ridden with lag, even with battle animations off, and not very good variety, and I just think some people didn't really like the Sinnoh region as a whole, at least when it was first introduced.
Platinum, however, was a major improvement. It did fix many of the problems with DP and made the game way more interesting. More variety, no more sluggish lag, more fun Gym Leaders and Elite Four, a deeper plot. And of course, the Battle Frontier, which is by far the biggest thing that keeps people from saying Platinum is just the same thing as DP other than the big event, for the most part. But I suppose that people still weren't big for Sinnoh as a whole then, maybe it was just what the region had to offer not being as much as others. But I like it for what it is, I can safely say that.
Then we get to Black and White. I'm always thinking people love these games for two major reasons: one, the story, which involves you chasing Team Plasma throughout mostly the entire game, and two, only being able to use new Pokémon. The thing with the latter is that they tried to make BW feel like "starting from a new beginning". But what I think people seem to dislike them for is the linearity of the Unova region, the abundance of characters that tend to get in the way, hating new Pokémon (like the starters), apparently hand-holding (which I never really found to be that much of an issue), and claims that they're the easiest games in the series (and i'm gonna get to that in a bit). And then there are people who DON'T like only being able to use new Pokémon. The thing with only being able to use new Pokémon is, when you start a new region, isn't that exactly what you want - to experience new Pokémon? Sure, people like variety, but I guess the only new stuff thing is something that's dividing. I didn't like it much at first, but it has grew on me (although i'll still take BW2 and XY's variety over it any day). As for complaints that they're the easiest? Horse s***. Admittedly some of the Gym Leaders aren't that hard, but look at foes like Lenora, Elesa, Clay to an extent, the abundance of rival battles the game throws at you, the Elite Four to an extent, and N and Ghetsis, god help me, and tell me this game is cake, given the small dex that limits your options. Seriously. Even the trainers (at least the higher classes like Ace Trainers and Veterans) I rarely found that easy, either. I guess I might be a bit biased here, and this is coming from someone who did a Nuzlocke on Black with only one death up until the Elite Four, and then lost half my team against them. If there's anything that makes them easy, it's the fact that there are so many powerful Pokémon, like Darmanitan, Haxorus, Krookodile, Chandelure, Excadrill and such, that are really tough, although Audino and the Lucky Egg make training easier. But on the bad side, there's also the lack of post-game activity, this I will admit. While not THE worst post-game in the series (not as bad as Gen I or RS), it doesn't keep you too entertained for a while.
And last, but in my case definitely not least, Black 2 and White 2. They are no question my favorite pre-XY games (even now I keep going back and forth between them), and there is a LOT to like about them, but sadly, I feel they get even worse hate than their predecessors. Why? For not being what they were. Okay, so that's basically saying people wanted it to be the same deal - chasing the team throughout the story, only using new Pokémon and fighting Ghetsis to beat the game instead of the Champion yet again? Is that what the people who hated BW2 wanted? Don't you think you WOULDN'T want the same experience? And that's what separates BW2 from Emerald and Platinum to me - the fact that they actually add stuff to the adventure, like new areas, new Gym Leaders, and actually a different story instead of rehashing most of it. And the post-game has lots to do as well, including the entire southern part of Unova, battling lots of powerful trainers, and cool stuff like the PWT, collecting medals, and Black Tower/White Treehollow. In short, I will never, EVER dislike them. And yet, it disappoints me that people don't realize just how much content they have to offer and some even say that they're the only games that do nothing for the series. Although personally, and I said this in that thread I made in the 5th gen forum, but I feel that there are several people here who weren't big for BW but REALLY like BW2.
And that's just what I feel I have to share for myself on these four games.