We're actively monitoring every dungeon and working on balancing issues we encounter appropriately. We'll be keeping an eye on bosses we think don't have enough varied mechanics to warrant their large health pools and updating them over time to make them more varied/interesting fights. We'll be monitoring, and continually tweaking/adding to dungeon rewards over time and of course balancing where we see the need. And of course, we'll be looking at adding more dungeons as well!
All of that being said, the game is VERY new for most of our players, and I can absolutely promise with more knowledge of the game and advanced player skill, the explorable dungeons can all be overcome by being skilled groups. We've seen many groups do it just fine in our internal alpha test once they had time to learn how to play the game well. Just like Domain of Anguish in [Guild Wars], it takes time and practice to learn how to overcome stuff as hard as our explorable mode dungeons, and that's exactly the kind of players they are designed for.