Well, from what I understand, not everything gets tied up in the first book, and hence won't be tied up in the first season either. This is going at least one more season, and probably a third to match the books. So some of the payoffs will come in other seasons.
I don't mind the flashbacks too much, as I think linear storytelling can also get boring and pedestrian. I think most of the complaints seem to be from people who expected or want less character development and more action. It's a very hard line to walk and I'm not sure the series will ever satisfy either camp completely. I think there was a lot more to the Setrakian flashbacks than just telling us "they met in a concentration camp." It was showing us what he witnessed of the Master and what he went through to survive, up to and including carving the Master's box. It's a shame that people have no patience for character scenes anymore.
As far as the reveal of the Master, I kind of agree. But I also didn't expect too much from a 1st season TV budget.
I've enjoyed the show very much so far, but it's never wowed me. But when the end of each episode comes, I do wish it would go on another half hour or so.