I'm not really sure what Blizzard is going to do. If you can believe it, they were much MORE OP in beta than they are now (you could solo those rock elemental elites west of ToT and end with full HP + RP unglyphed - it was ridiculous). I personally think they need to nerf the class quite a bit, but I think they may wait a while before doing it. So I'd suggest to level one ASAP if you are going to level one at all.

With respect to PvP, they are cloth caster killers. Your only chance with a DK as an affliction lock is to stay at maximum range and kite them (CoExhaustion). If you get within reach of Death Grip, then you are done for. If you are Demonology, then you'd better pop meta and felguard stun, fear, deathcoil them immediately. If they get on you, they will mind freeze and strangulate and generally shut you down and destroy you right away. Be sure to pop your shadow absorption if you get a chance. Most are unholy spec and do a lot of shadow damage (e.g., Scourge Strike). The smart unholy ones will leap the ghoul at you and hit 'Gnaw' which is a pet 3 second stun. They'll get range and that's that. Many also have a +15 run speed, which won't help things.

Did I mention the class is currently OP? :-/