In the early MMORPG's, zones were level fixed, so there was a beginner zone, an intermediate zone, a high-level zone, etc. As a new character, you couldn't go to the high-level zone without getting instantly killed. Once you out-leveled a zone, there was little reason to go back.
Some games had a mix, so that a low-level zone might have a camp of higher mobs in it.
New games, like ESO, have made it so that all zones appear the same to the character. This means a new character can go almost anywhere, and you can't out-level a zone once you finish it. But it also makes it kind of monotonous, since you can't venture into a super dangerous higher zone for fun.
My optimum case would be zones that appear the same, except with a caveat. If you are lvl 10, for example, and go into a higher level zone, say a lvl 50 zone, the mobs appear +1 level to you for every 10 levels. So a lvl 10 character in a 50 zone would see +4 mobs. That preserves the danger of going into a zone that is too high, but still allows you to explore everywhere.
I also like the mix, where you might suddenly stumble upon a camp of higher mobs in a lower level zone. So even though you are in a lvl 10 zone, with a lvl 10 toon, you might encounter a camp of +4 mobs.
2020: 43 years on the Net.