Frankly, I can't see how people can call "exploration" the act of following passively a quest mark on a compass/minimap.
Well... It isn't exploration when you just brainlesly follow a quest marker, I agree. What I call exploration as far as I'm concerned is the ability to ignore the quest marker to go do whatever you want to do. Which is why I think the ES games are true open worlds while MMX, D:OS, BG, etc are only "half open worlds". You can't go do whatever you please, either because you need to unlock a special ability first ( especially in MMX, Metroidvania syndrom ) or because, as mentionned by Artemis, you are rewarded with a club to the head anytime you try to wander off the beaten path and into territories you aren't actually suppose to go to at this point of the game.
I also said in another post that having limitations helps create a more realistic world where there are actual dangerous zones, while Oblivion/Skyrim do not features that kind of dangers. "Oh god, this is undead territory. Man, I'll just find a way around". This sentence is what makes half-opene worlds, well, "half-opened". You can go there, but you can't. On the other hand, when you come back with a vengeance and/or are able to beat the encounters there despite not being technically ready, you feel great. But... this isn't as open-worlded as a (modern)ES / bethesda's FallOut game.
At the end of the day, when you're done exploring and adventuring and you want to go back to the main quest, though, I'm always glad to have the possibility to get a quest marker, just to remind me of what and where I was supposed to go. Taking notes on a sheet of paper as you go is so 1990... Not all modern features are bad.
What you call true open world I call boring wandering around
I'm not sure what you actually call "exploration" then, to be honest with you. I guess that's something that, maybe, got multiple meanings depending on one's past experience ...?