The arena gear is decent for tanking. The reason to stack defense skill rating from gear is to attain the 'Un-Critable' status. The reason to use defense skill rating over resilience is because resilience does not give you dodge/parry/block/miss %'s. But arena gear has HIGH stamina on it. So it's feasible to tank with it on, for 5-man regular instances just fine.
It has 6 sockets for the full set. So with 8 defense gems (ignoring socket bonuses) that's, 48 defense rating ~ 20 defense skill. You'll want more than that. The 144 resilience from the set is nice. That's 3.65% chance to reduce taking crit from resilience. 0.8% from defense, that's only 4.45% to reduce crits. And only 0.8% dodge/parry/block/miss.
I'd say run some instances that are lower level first. Blood furnace, slave pens, auchenai crypts for example. something that isn't quite lvl 70 mobs. Get some better tanking gear. in fact, you can get most of your defense rating /block "value"/parry/dodge/sta gear from questing. Or buy some
felsteel gear from the AH to supplement the gear.