lavish production/stunning locales, great casting (overall), a surprising degree of historical accuracy (the fact that a significant number of the individuals actually existed) and the scenes of raids both in GB and on the continent.
however, there were/are fairly lengthy static periods as related to plot/character development- overmuch in my opinion. i understand you couldn't have armed conflict/seaborn action/etc. every episode, but i think the story bogged down from time to time in material unnecessary to furthering the story.
and one last thing. the undisputed central character midway through the show- Ivar (as an adult)- may be a gifted actor, however, for me everytime he opened his mouth all i could see was "Fez" from "That 70s Show".
if you don't believe me watch about 5 minutes on the two characters and see if you don't agree.