Well I'll give it a shot!
-S3 tried to be both funny and scary, but was neither (S4 at least was very humorous and entertaining, even if you don't think it was scary).
-S3's chase scenes felt by-the-numbers, dull and like Wes Craven was bored by the whole process--which he was. He made Scream 3 solely to get financing for
Music From the Heart. At least in S4 there was an edge to scenes like when Olivia died or the Kirby-Charlie scene that left audiences literally upset that she died. Which leads to....
-S3's new characters were all dull and vapid characters that while witty parodies of Hollywood archetypes (sleazy/hedonist producer, whiny-hack director, self-centered diva movie star, two-faced ingenue, etc.) were utterly boring and unlikable thus making them dying pointless and uninvolving. While Trevor and Olivia weren't ones to write home about, Kirby was the first sequel-created character who lived up to the original cast and then there's Jill and Charlie....
-S3's villain motive relied on a
Days of Our Lives revelation in that Sid had a long-lost half-brother who is evil and wants to kill her....and who then retconned the near-perfect first film by revealing he manipulated Billy and Stu into killing Maureen, frame Cotton and then everything else. Meanwhile, Jill had the best motive since Scream 1 in being the poster child of the Facebook/Twitter/Jersey Shore/Kardashian/Me Generation.