It takes a lot of stacking of all of the stats to be effective. Currently I'm at 26% dodge, and about 20% parry w/ only Horn of Winter up, with 3/5 T7, then mostly badge/heroic/naxx drops. I have mostly all dodge/def or dodge/stamina gems because of the lack of mitigation that we have. The stamina helps to suck of some of the overall damage that take as we have to basically absorb the damage like a druid would.
The biggest problem that I've found for a death knight, is stuns/web wraps just OWN US totally. So the extra stamina will help with survival of above said actions. At the same time the extra avoidance helps a lot with preventing the spiky damage that we do take. It's not abnormal for me to go from full health to about 70% in 2 rapid hits.
We were doing Naxx the other night and we were on Maxx the large spider at the end of the spider wing. At around 15% health left, the MT dies, so i went from spider duty to Boss Duty, i survived the first web wrap thanks to Iceborn Fort, I then survived the second web wrap thanks to lichborne. Needless to say, i didn't survive the 3rd web wrap.
In short, we have to stack defense, avoidance, and stamina to be good at our jobs elsewise we find out selfs too squishy with high avoidance, or too much health with little avoidance.
DKs are all about balance. Stam and avoidance are both important. I prefer to use dodge/stam gems in red and blue and def/stam in yellow(if you need the socket bonus). If he is having threat problems in this content exp/hit gems can go in yellow sockets too but most of that should just come from gear.
I don't consider myself a great theory crafter, but with just heroic naxx gear about 2/5ths valorous 1/5th heroic set and frost spec'd and on a boss where all I'm worrying about it not getting hit I can pull 75% avoidance with next to zero avoidance gems. ( I think this is ridiculous for T7 level gear and expect this number to be nerfed before Ulduar )