i would still assume the game is balanced around the "normal" or average player, who finds the obvious stuff but may miss some of the well hidden things. There is also a chat, where people will ask how to do things, and if you like it or not, they will get their answers.
the normal or average player will probably not get the maximum amount of xp, and maybe the explorer/completitionist will neither (because this would involve amok-killing every single npc after every quest is done)
so if you don't explore at all, brute-force every door and only do the main plot, you may end up with very hard encounters. I don't think this will happen to a lot of people, because either players like this don't enjoy RPGs at all, or they will find solutions and guides to find all the stuff they want to find.
and if you explore everything and get every XP you can get, you may outlevel stuff, but not that far, because you can't grind xp, and doing quests and finding the important things is more rewarding (in the sense of giving you more XP) than finding hidden chests or basements or puzzles.
of course there are different ways to solve quests, and yep, there are ways of doing things that give more xp than others, but the difference is not game breaking, at least i hope.
