Hmm...
Good -
-I reckon the character development is much better. For the Pokemon at least. I think nearly every Pokemon has had some, of course, some more than others. With the trainers...it's hit and miss. Although like with the Pokemon, nearly every character has had some.
-Actually good gym battles. Most of Kanto and Johto's gym battles were a joke. Hoenn improved but there was still some dues ex lying around. Sinnoh has much better battles, all badges have been fairly obtained for one thing, and there was no crazy hax. (I'll put Ash's victory over Crasher Wake down due to competence).
-Gym leader character and abundant appearances. Obviously something that lacked, gym leaders would have an average of two episodes, at the most three. Sinnoh has had a very good spread.
-References to previous sagas!
-Introduction of excellent characters. For example, you have recurring rivals like Paul who has an army of fanpeople lining up for him. Then you have the lesser recurring characters that are still very enjoyable, such as Conway.
-The Pokemon have excellent characteristics too. We've certainly never seen anything like Croagunk. Then you have the emotionally scarred Chimchar/Monferno, the cowardly Gligar/Gliscor etc.
-Lack of fillers. Whilst unlike most other people here, I enjoy the odd filler, a saga is obviously better off if it gets straight to the point.
-Competent villains. I dunno where Hunter J came from, but she's awesome. And Team Galactic seem to have a thing going on (let's just hope they don't screw their finale up. Aww hell, we all know they will).
Bad -
-Piplup. Nuff said.
-Dawn's rivals lack substance in comparison to May's rivals.
-Lack of Team Rocket Pokemon character. It's rare when Mime Jr. even appears, which is my greatest annoyance. It seems a Team Rocket Pokemon's best episode is when it leaves, a.k.a. Dustox.
-Reusing of previous plots. But that's what you expect in an umpteen-episode show.
One thing I have secretly enjoyed is the constant fan speculation of captures and evolutions, and outrage if there is a filler. Not to mention the Aipom/Buizel fiasco. It provides the rest of us with numerous popcorn-deckchair moments where we sit there thinking that at least our minds are superior to some out there.