This is probably pretty far out in the feature creep zone, but thinking about it, a frozen water body seems to provide lots of potential for... stuff:
It's slippery, creating a challenge in manoeuvrability.
It could break, plunging your creation into the depths.
It could melt from fire, see above.
Uneven thickness (strength) would make for some less reliable and more dangerous terrain.

I recall reading the theme being somehow related to Vikings, too. Ice is a Viking thing, right?