The thing I'm most impressed with is how diverse and detailed the world is. If you only travel by horse and roads you'll miss a LOT! I jog to new areas generally taking the scenic route and I've found all kinds of things. I really want to explore that huge tree on a mountain in south velen, but its still above my level. And there's just so much loot! Yeah a lot of it is not terribly useful, but there's always something to do with it. I really like that nothing is strictly 'junk'.
I've walked up by novigrad but haven't been in yet. Now down in south velen with the army camp. I should probably just go ahead to novigrad here shortly as I'm running out of level ability to clear everything.
You'll need to be Level 27 at least before trying that. Trust me. :whiste:
I've completed III and I can confirm you can still explore the world after. The main quest section vanishes, but everything else is still there.