It's a big issue with playing PvE and PvP with same build as shadow. In PvP build you want to skip Shadow Affinity and Silent Resolve because it's useless for PvP and it costs you 8 talent points. However with out it you may overaggro without even using Mind Blast at all.
This is the build I use for PvP. I use it for clearing dungeons aswell. But without tankiness you won't be able to do much with it. http://db.vanillagaming.org/?talent#bxzGshZZVMGdctRt