Yea same for me, 2-3 seasons of nothing intresting. Every time something intresting would pop up it would sink my expectations little by little. I don't know how many of the anime these past few seasons I've watched just to keep up to date; because I started them or just to keep with discussion.
Ah, really Nagisa isn't useless. She really an another person at the end of the School part and especially in AS-part, she changed enough. It's a one of the main ideas of Clannad about changing with help of others, but only some people can see it.