First: ohmygod the costumes
Second: Jonathan Rhys Myers is never who I would have picked out for Henry VIII, but he plays him incredibly well. He channels the King's brief attention span and egocentrism perfectly.
Third: The Tudors plays fast and loose sometimes with history, but I'm willing to give it a pass, for the most part. The show has very few pretensions to realism, and what has been changed doesn't feel out of place. This is a look at the political machinations of the era, and if the show is willing to do a little hand-waving for an impressionistic picture of the whole, I'm willing to forgive it.
Fourth: This show is my favorite visual adaptation of 16th century England, a crowded genre of which I've seen a few. The 1998 Cate Blanchett vehicle Elizabeth is probably my next favorite, along with it's sequel Elizabeth: The Golden Age. The unexceptional Elizabeth I, the 2005 TV mini-series, and A Man for All Seasons, the 1966 film adapted from a play, fill out the middleThe "We watched a History Channel documentary and that's all the research we need" Shakespeare in Love far in the back of the pack.
Fifth: Beyond Rhys Meyers, the rest of the cast is great as well. Natalie Dormer is a personal favorite. Sam Neill plays a convincing Cardinal Wolsey, and Peter O'Toole is a delightful treat as Pope Paul III. The lesser-known actors they get for Sir Thomas More, Catherine of Aragon, and Thomas Cromwell are also very compelling.
I thought this show would be a guilty pleasure, but turns out there's no reason for adjective at all. It is simply a pleasure, and I'm enjoying it immensely. I've finished season 1 and am about halfway through season 2, and I plan to continue for a while.