I rarely give a rating of 10, there would always be some part that would be missing and something goes wrong in any department, nothing ever went wrong with this.
The whole first season is one murder case, so you can expect some slow pace, and slow pace doesn't always mean a drag, and "The Killing" is the best example for that, the pace is deliberately set to slow, to let all emotions, settings to sink in, the show gives you very subtle clues and you would swear that know who killed "Rosie Larsen" and think it is your idea, which is most often first proved right then wrong :) The acting, direction, screenplay, music a absolute top notch, a lessen that other crime thrillers (CSI, Criminal Minds, Suspect Behavior et. al.) should take a note of this epic of a thriller.
If you haven't watch it yet you are missing an epic.