I have to say that the identity of the serial killer in Dexter is not tantamount to that most important thing about TV shows: characters. Dexter is so great and successful because the characters are fantastic. So a plot twist wasn't all it was built up to be. It's not about the twist, it's about the journey and the characters you are on that journey with. All of my favorite dramas (be it Battlestar Galactica, The Wire, Sons of Anarchy, Farscape, Dexter or True Blood), I care about the characters first and foremost. The only episode I truly did not like of BSG was simply because it took a character to a place that didn't feel natural.
That is what I'm hoping Boardwalk Empire will bring to the table, as great actors, a great director and great production values for a good story can only get you so far. We need great characters as well. However, let's just wait and see before we start grading it against other shows.