He has ruined Dexter!
Dexter was a great show, but i doubt any guest actor could top Lithgow's amazing performance. The earlier seasons were good, and the last season was good as well, but i can't help but to compare it with the Trinity season, and i doubt anything could make a season as creepy as Arthur did.
Don't get me wrong, i am probably one of the few who actually thought the new season was pretty good, and i even liked Lumen's performance, but it did seem to me the writers had trouble creating an equally creepy antagonist, so they took the easy way out and made it a group of killer slash rapist guys led by a stereotypical sociopatic mental coach (a group of people who are creepy and untrustworthy by definition if you ask me). This way they tried to approach Lithgow's characters creepyness by tossing the most evil types they could find in, but it did come across a bit sought to me.
Therefore i dare say that Lithgow has ruined all other seasons of Dexter, because i can now only see the show in pre- and post- Trinity seasons which will probably never come close to the sheer brilliance this guy brought to the show. Even if Dexter is still one of the best shows on TV.