My apologies, I had just assumed that was a given, but you know what they say about assuming . . .
Ya, Cannot Be Frozen is pretty much a need for any character.
Ulmo is technically right. Every character hits a "wall" at the end of NM and cannot really do well in hell until they get some decent gear. For melee characters (Bowazons as well) they typically have a harder time with this, 'cause not only do they need to find decent stats on a weapon, that weapon also has to be a high end exceptional or elite item.
A caster can find a +20% FCR, +2 skills, +100 mana wand/orb on any type of wand/orb, and still rock hell with it.
Although there are items, like Kingslayer (MalUmGulFal) and Crescent Moon (ShaelUmTir) that are not extremely rare, but decent enough to boost your damage.