The general theme gives the potential for a whole world of plot possibilities. The characters are very engaging, specially the buddy thing going with Jeremiah and Kurdy (Perry and Warner). Marcus, Mr. Smith, Lee, Erin ... all the characters were interesting. Even the extras, or the bad(?) guys. It's not just about surviving ... it's about moving on, hope, and building a future. Most of the time, it didn't come out too preachy, but JMS shows somehow tend to be that way.
Each season has an underlying plot that connects the episodes together, but each episode has it's own story. It has a powerhouse season ender for the 1st season, and a powerhouse opener for the 2nd season. I liked all the episodes, although the last episode of season 2 (of the series, in general) felt rushed, as the writers knew they had to end the season abruptly.
Acting was pretty good, but sometimes (not often), the dialog seems a bit corny, which makes it tough to act out with credibility, hehehe.
I was real disappointed that this wasn't renewed, while more shows with much less story to tell last pretty long. This is an excellent show that not enough people watched. If it somehow gets a 3rd season, I'd continue watching it. :)