When Enderal first came out, I explored everywhere I could, except the areas that were blocked/locked. Then when I had done all the side quests, I did the main quest. No problems. I think you cannot break a quest by exploring as anywhere that could happen is locked until unlocked by a quest, or in some cases blocked by an invisible wall. [You occasionally find a door that tells you it cannot be opened without a key and cannot be picked. Sometimes that means you must find an actual key somewhere, and sometimes it means it will automatically be unlocked when the appropriate quest is initiated.] One thing you will discover is that you might clean out a dungeon while exploring, then a quest will take you back there and the "boss" or quest item that hadn't been there before now will be.