I love the casting of Jeremy Northam (Sir Thomas More) and Sam Neill (Cardinal Wolsey).
I read an earlier comment after I had already expressed the following thought elsewhere, and I completely agree -- Steven Waddington (Buckingham) would have been a better Henry VIII - he's bigger (he properly fills the screen, which in various shots J R-M painfully cannot, either in height or breadth); red-haired (as Henry was); and a POWERFUL, mesmerizing actor who's a better age for the part. (J R-M's eyes are riveting, but that's not enough for the part b/c at this stage of Henry's life, his fame was largely due to his physical dominance, learning & musical skill.) Showtime seems to be trying to appeal to a VERY young, VH-1 audience with the J R-M casting. Or, as they suggest, to people who don't know the story.
That's my second issue - don't suggest in the ads that you're going to tell the REAL story when you're not. Some dramatic license is expected (like flipping France for Portgual b/c they introduced Francis I early on) but there is no GOOD excuse for making a composite of Henry's sisters by telling Princess Mary Rose Tudor's story, but calling the character Princess Margaret, which was her older sister's name.
The real Margaret had a dramatic story, too -- and she's got the line to the current royal family through her great-granddaughter, Mary, Queen of Scots -- but they lost the chance to tell that by combining the sisters. Presumably they did it b/c they thought the audience was so dumb that we couldn't handle Henry's daughter and sister both being named Mary. Too bad.