Well, he has that british mixture of propiety and passion and danger that made me go week in the knees when I discovered him in in the period drama North and South... Although I must confess that it wasn't just that... He is also a very handsome man, tall and dark and with the bluest eyes I've seen on screen (yes... even bluer than David Hewlett's).
Another British Hunkie. Excellent.
I have watched North & South so many times now I know the lines off by heart - although my Northern accent is more genuine than the vast majority of the cast ;-) Richard Armitage brought John Thornton to life for me in a way I thought no actor could. For that alone he has my undying devotion.