I realized it was because Irons plays him like a more sensible fellow than most of the actors I've seen playing him. Irons plays him like the rock Elizabeth needed. Always there, except for the occasional marriage that got in the way. I think I liked him so much in that role because he played Leicester the way I always wished he could be played.
But you know, Jeremy Irons usually plays that sort of character, the sensible, intelligent, dependable one with the wry sense of humor.
I have loved Irons in Kingdom of Heaven and Eragon, and he was even good playing Scar in The Lion King.
Anyway, very satisfying movie. I even forgave Helen Mirren for not being Glenda Jackson.