The episode was well put together and well paced (the dinner scene was incredibly tense), but I had problems with it on another level. Before, Arthur served almost as an inspiration for Dexter in a way - a teacher figure who he could learn from, how to balance being a killer with a successful happy home life. Now we learn that he's a monster at home too and the happiness is a charade maintained by everyone else's sheer terror. It removes a lot of the shades of grey and the moral ambiguity involved in robbing this loving family of their loving father. While the wife and daughter's reactions to Dexter holding a knife over Arthur in the kitchen makes it clear they do actually care for him (Stockholm syndrome?), it still removes a lot of the more difficult questions by making him just "a bad guy".
As for the ending twist, it was effective in shocking me, but it strains credibility quite a bit. I hope they deal with it well...
As for the subplots (Rita/Elliott, LaGuerta/Batista)...nope, still don't care.
On a final note, major major props to Michael C Hall and John Lithgow, as always. I really hope Hall gets at least one Emmy before Dexter ends, but I know there's always tough competition.