Can you discard locations / enemies which have the text "Immune to player card effects"?
Can you discard locations / enemies which have the text "Immune to player card effects"?
Other rulings have made it clear that looking at a card doesn't change its location: i.e., if you look at the top card of your deck, you can still use card effects that draw (or discard) the top card of your deck while looking at it. And I think there's been a similar ruling about the encounter deck, though my Google-fu is failing me right now to actually find those rulings.
Therefore, when you look at the top card of the encounter deck, it isn't yet in play, and so its text hasn't taken effect yet. Therefore, I think the right answer is "Yes, you can discard the card that you look at if it's of the right type, whatever its text says, because its text has not yet taken effect."
Any cards that are vital to the quest (like boss enemies that you face in quest stage 3) have probably been "set aside, out of play" already, and aren't in the encounter deck to be discarded, so you won't be able to discard them. But any "immune to player card effect" cards that are actually in the encounter deck... yeah, they're fair game to Celduin Traveler or Ithilien Lookout.
Cards in the encounter deck are not in play, so yes, there imunity (it's like a passive effect) doesn't apply (there was a ruling for this).
In fact, it can be cool to discard a location or an enmy that is... immune to player card effects!
Edited by LecitadinGreat for Great Barrows!