For a little while now, I've been playing and enjoying Terraria, a side-scrolling exploration game (somewhat similar, from what I hear, to Minecraft). Its 16-bit vibe really hits the nostalgia button for me, not to mention that it's just plain fun.
That said, I think the single most attractive feature of this game is that you get to really explore a computer-generated world (no two of which are alike!). It's the exploration factor that attracts me most. Once I hit the "hard mode" portion of the game, it starts to feel like a grind to me. The discovery of brand new places and items is my carrot on the stick; once I've fully uncovered the map, the game loses its luster.
I think the same thing can be said for a number of other games I have enjoyed in the past, including ones like Skyrim. The expansive world is just plain fun to explore; there's always a new cave, or city, or ruin to find and explore. Quests can keep things interesting, but it's seeing new places that really gets me excited.
Are there any games out there that are, to some degree, solely about the exploration? I'm pretty sure that Dear Esther fits that bill (and I have yet to play it), but I'm wondering if there are others I've missed. I have to believe that purely exploration-based games have a market (see Beyond Eyes, for example). If anyone can provide recommendations for titles in this space, I'd love to hear them.