I think the dividing line is when it starts to take 'away' from the site. Flashy designs are too much if it distracts from the content of the site. Intros are too much if people can't skip them if they want to or if it turns them off to eh point of not wanting to be on the site. Animated dropdowns, etc, become too much if it slows down the site or even if they've put so much time into them that they didn't bother making it easily navigatable. It's all fine and dandy to try to make things fun...but if it starts to take away from what the site is all about, then there's just no point.