Late to the party, but signed in for the first time to counter a lot of the truly awful advice on this Dominator forum from people who seem to only play brutes/scrappers and nothing else
If you're building a dominator for defense, you're wasting your time and should stick to playing Brutes. The beauty of a dominator is its ability to lock down mobs, especially when you've achieved permadom. A mezzed enemy deals no damage. My main is a mind/fire dom, as it was back on live, easily solo's taskforces, story arcs, trials etc. Build a dom for +rech and +acc so you can use your powers to their best. If you're dying often, you need to click your mez powers once in a while
The generic advice spouted of "bUiLd fOr SoFt CaP" is poor advice from people who don't understand Dom's and don't know how to play anything other than a melee fighter