Still no explanation as to why Andy was immune to Maryanne for so long. Tara's mom and Hoyt, plus the kids, were the only recurring townspeople unaffected.
Maryanne only really took interest in someone if it suited her. It wasn't that they were immune, just it wasn't to her benefit. PLUS she said she peeled away their inhibitions and let them run free. Considering Andy was drunk for much of the season, his inhibitions were pretty much stripped away already. The kids were, well, just kids - what's inhibiting them? As for Tara's mother - she had found devout worship in God; maybe Maryanne thought that was enough...?
The season finale was a bit iffy. Maryanne's demise was quick and wrapped up too fast. The vampire queen is still a terrible actress. Bill is still the most boring actor on the show, and I don't blame Sookie for turning him down!