1) Bad guys got a lot better looking. Zancoon (or whatever his name was) to be sure. But not just main bad guys, also the extras who just stood around until being vanquished.
2) Alyssa Milano continued to vary her hairstyles while Holly Marie Combs continued to demonstrate Piper's steady ground by keeping the same hairstyle. But best hair of the season was Rose McGowan, especially half-way into the season when she added bangs.
3) The season probably belonged to Rose more than anyone else. Alyssa got to deepen Phoebe throughout (being the one Leo went to to explain how the Avatars had destroyed everything and her realization that her boss deserved to play hooky as well) the season. Holly seemed forgotten by the writers. Piper was involved in the action but she was often pretty much the same. Maybe that's supposed to show she's steady? But it didn't give her a lot of different aspects to play with. Rose was lucky because the writers seemed really interested in Paige for a turn. So we got to see her stand up to her sisters, we got to see her frustrated with running magic school, we got to see a light side and a dark side.
4) The elders really are little s**ts. It was never more obvious how pompous and judgmental those supposedly watching out for us were then in their attacks on Leo and stripping him of his powers. Piper's fury with them was not just consistent with her character, it was the only response to the crap the elders pulled.
5) Adult Chris is sorely missed. With no powers for Leo, they especially needed a strong male.
6) The show's visual look was the best this season. Especially in episodes directed by Levar Burton. That's probably most clear when San Francisco freezes as everyone falls asleep.
"Iraq snapshot" (The Common Ills):