Good game. It's far too in depth for me to enjoy it completely, but many games are for me these days, sadly. It is indeed a massive plethora of potentially hundreds upon hundreds of hours of nothing but exploration and mystery. When the game originally came out, my friend said an odd comment that stuck with me and still describes the game perfectly for me; he said "The thing I love about this game is that you can spend hours doing quite literally nothing to actually further the game and not know it while you're doing it". Like you said, you can spend so much time just exploring a 'little' (in comparison to the game world) nook, find some random treasure and/or powerful opponent...or perhaps nothing at all; but even if you find nothing, it's still exhilarating somehow.
For me though, I really don't have the time to rack in hundreds of hours into a game anymore. I would love to be able to, and hopefully will be able to again someday; but for now those days are behind me and I've still only got about 60 hours in Skyrim. Which, for me, is unheard of in the past few years. It's a testament that I finished the story, but I realized the real game lies in it's ability to continue on after that plot ends. It's almost like an adventurous GTA, where instead of just driving around a city and fuckin' around with cheats, I'll dive in with the insane amount of mods there are and have fun that way.
I still jump in from time to time to fly around as Goku and have Iron Man as a minion, just fuckin' up those dragons and stealing their balls, hoping they bring me wishes.