If u like grouping alot, go for deep frost with the the 5/5/5 base and ull be ok.
I tanked UK, nexus, AN and so on perfectly well without any def gear, just high stamina gear
Also, what i see alot of people saying is that its hard to get agro on groups while tanking as frost, i think this has something to do with the pulls, cause i hardly ever have trouble..
What i think is the way to go is the following:
IF there are casters in the group u run to them, else u let them come to you
Open with Icy touch, BEFORE they seen ur groupmates, this gives u like 2 seconds, enough to get more skills off
use plague strike while taking a few steps towards the charging mobs
hit Pestilence, this alone should mean the healer can start healing
Hit howling blast, preferably with icy chill or killing machine buff up
Hit Blood boil
This should give u a 10k headstart on the the secondairy mobs, and with 1-2 melee swings + icy and plague damage aroudn 6-8k extra on main target, Dps should be hitting any damage before ur pestilence hits, and with enough rune power generated for 2 Frost strikes you can make sure the mob stays on you