As Chaos Rush said,

based on the pokemon sprites, it looks like they were lazy...
They're either the same as D/P/P, or look worse than D/P/P

I don't get why they bothered to make new sprites for pokemon like slugma that you don't see until you're in Kanto but Gen IV evo pokemon like Mamoswine don't get new sprites.

Well they look somewhat lighter and the outline is better mostly, imo.

*Note: This is purely my own personal opinion.

Johto GYM Leaders:

1)Falkner: Looks cool to me.
2)Bugsy: Okay-ish.
3)Whitney: Cute. Great pose, too.
4)Morty: VERY cool.
5)Chuck: Good going, strong man!
6)Jasmine: Not bad, though nothing special. Also, her hair from above (the 2 bundles sticking out) are kinda long.
7)Pryce: Cool outfit.
8]Clair: Most awesome sprite of them all, though her colour is a bit too blue-ish.

Kanto GYM Leaders: (Not in order though)

1)Brock: Nothing special, except that the orange jacket looks a bit weird on him.
2)Misty: The blue-and-white outfit looks cool on her. However, her hair looks a bit awkward.
3)Lt. Surge: Looks cool to me.
4)Erika: The same old Erika. Looks alright to me.
5)Janine: Great pose. Great outfit.
6)Sabrina: What's with Sabrina's new sprite? I didn't even recognize her! Her hair was too long, but now, all of a sudden, it's shoulder's length. Also, she doesn't look so 'dark' anymore; her old outfit was better. I don't like her much, but she's not that bad either.
7)Blaine: An old man trying to look cool? Not really working...
8]Green: Slightly disappoints me. Though his hair is longer now, which makes him look cooler, I liked the old blue outfit more than this one.

Comments on some of the Pokemon sprites:

Ivysaur: Since when was his colour blue?
Raichu: His face doesn't look right to me for some reason.
Persian: I never knew some Pokemon's body could actually shrink :\
Psyduck: Eyes are too wide.
Scyther: His head is small and looks kinda flat.
Magmar: His head is smaller now, which makes him look weird.
Pinsir: HORRIBLE.
Vaporeon: His tail is waay longer than it should be.
Jolteon: I don't know. Something's weird about him. His older sprite looks better.

For the most part, I think that most of the sprites aren't too incredible, and a couple just look bad compared to the ones from D/P/Pt. The way that were drawn has a different style it seems, as the sprites look more precisely drawn, making some pokemon look rather frail. Plus, a lot of the pokemon don't emphasive the power and characteristics they have. However, I still think that a lot of the sprites were well done for this game, such as Muk, Kabutops, Scizor, Raikou, and Gyarados. Saying this though, there are still the ones that don't look too great such as the obese Tyranitar, and the cross-eyed Lapras.

Raichu has horrid shading....I swear that rat will never get a decent sprite. .-.
Also ampharos looks tubby. :V All I really want though is for them too give floatzel a new sprite wears hes not all like "WAZZUUUUUUUUUUUUUP?!"

Sabrina's is the only one that confounds me. What is that bizarre wristband for? Why does she look younger all of a sudden? Is she even the same person?

Other than that, not many concerns.

Im just glad for the change in sprites period.

Sabrina's new sprite makes her look...Hrm. Back then, she dressed as a Japanese medium, didn't she? Now, you wouldn't really be able to tell that she could bend a spoon. I think her outfit was inspired by Flannery's clothing style.

Shuckle's is improved, and that's all that matters.
Lite, I think I know what you meant by "Hrm." ^-^