When I play a game that places meaningless objects all around the world for explorers to find, like banners in Assassin's Creed or intel items in Call of Duty 4, it feels like non-explorer designers throwing us a bone. They want us to play their game, but they don't really understand goals outside of achievement.
Exploration isn't about being able to walk off the beaten path, cover a lot of ground, or see every nook in a map. Exploration is about new experiences and creativity. The size of a game isn't as important as variation and dynamics. The number of skills isn't as important as how free we are to use those skills in fresh and personalized ways.
Hidden objects can be fun, but don't just copy and paste the same item into a hundred random places. Put some thought into it. Individualize the objects and place them in a meaningful way that tells a story.
In Star Wars: Galaxies, I once stumbled upon an ancient ruin in the middle of a forest, far from the cities. It was a great surprise. The ruin raised many questions in my mind. "What is it?" "Who did it belong to?" Unfortunately, there was no backstory to discover.
Arkham Asylum has two types of hidden objects. First, you have the Spirit of Arkham stones which unfold a story in a linear fashion. I finished that tale after beating The Joker, and it was the perfect way to end the game. But these stones were about achievement more than exploration. Second, there were clues related to villains not present in the game, like Catwoman and Harvey Dent. Those are a great example of what I mean by objects telling a story. Toys scattered on a bench, a campaign poster on a wall, a tea set -- stuff like that invites players to imagine how it got there.
Anyway, my point is that satisfying explorers involves more than just making us run around and find a dozen copies of some meaningless object.