Run "viking helmet" into Google Images. http://www.a2armory.com/images/leatherarmor/vikinghelmetleather.jpg It makes sense-- more of the face is protected, but you don't restrict your vision by having to look through a tiny slit the way the Ebony armor was.
I think it's kind-of funny how people picture Vikings with huge horns on their head. From what I've read they didn't do that because it gave enemies something to grab onto, they think that Greek Historians added that for extra "barbarian" effect.
The "Slits" are more suited to combatants in Arenas, and Knights under a Lord than they are Warriors, Warriors needed to see in the battlefield, and couldn't have their view only be a little slit. Knight Helmets had many slits, but not enough for somebody to attack you in the face. Knights usually served lords and kings, so they weren't running around in the wild trying to fight bandit-gangs and battling other knights like "Knight Errant" were. And I'm assuming they didn't wear big clunky helmets that obstructed their view wherever they were going.
Anywho, armor was made to protect you, not to look nice, unless you were rich enough to afford it to look nice... and even then many people didn't want to sacrifice protection for looks.