This season wasn't as good as the first season, and there were a lot of unnecessary additions such as Max and her step-brother. Bob was basically there to be a convenient plot mover, and since it seems the show wants to eventually have Joyce and Hopper hook up, they killed Bob off when he wasn't needed anymore. I agree that episode 7 was a letdown, but I think this group was brought in to give us a glimpse of possibly where season 3 is headed. I'm not sure how much more the show can pit the characters against the evils of the upside down since we've just seen two seasons of them battling it and winning. I feel another season in this same vein could get boring.
I foresaw the setup of Dustin befriending Dart that would lead to a later occurrence in which Dart would help him in some way, but instead they just had it where Dustin lays candy down for him and then he just walks by him. I thought this was a lame payoff for the build up. Also, I don't get why Dart had an individual personality of his own when it seemed as though everything from the upside down was connected in one form or another, therefor having a hive mind and hive senses, I suppose. The writing seemed to be a little too loose with this aspect.
Either way, the show was still kinda fun to watch, and I still liked the characters and their interactions with each other. I'm not saying it's a bad show, just that it seems to be slipping now.