If doing the thing where different enemies or areas are more challenging than others, one thing games could do to let players know what is a challenge and what is not is to take a leaf from tabletop role-playing's book and simply let a player know anything their character would know. So, when you come across a sort of enemy you've never seen before or you're in a certain area, an icon would show up on your screen, and you could pause the game and access some information about it. And if you come across something your character would have no way of knowing anything about, the thing would say, "Shit, you don't even know what this is, you may want to consider running or something." And maybe the game would remember what you had seen of the thing, so you could use that information to do some in-game research about it, if you wanted to. And for things that your character doesn't know everything about, but that he or she could make some assumptions about, you would get your character's best guess, and it would, of course, indicate how accurate your character believes the information to be.
This is actually done in ADOM, where your character observes what the enemy does (like 12 damage in an attack or has fast speed) and every time you look up information on the creature, it will tell you all the statistical data your character has collected. In all honesty, though, if you don't see that Greater Moloch doing any damage to you, and have no idea what it is, you're probably not going to live very long to know.
So, it has good things and bad things about it. I'd say that it is a nice thing to do in roguelikes, where you learn from your mistakes, anyhow. I think it could also work in an RPG, too.
Of course, ADOM also had a rather annoying way to level scale. There were a couple different things in the game, but the most notorious one is the uber-jackal. Basically, the more of a specific monster you kill, the more they level up, through what can only be described as your own personal natural selection. This can lead to relatively weak monsters, i.e. jackals, punching through your adamantium armor in packs of 20. Also, I think monster got stronger over time, as well (though the world was being corrupted, so that was plot-related.)
As for level scaling... meh. It really is lazy. I'd say you're better off with adding monsters where monsters should logically be. You shouldn't add a monster just because you suddenly hit level 5. That doesn't make sense. You shouldn't have monster level up because you did. That also makes no sense. You SHOULD have monsters scale with the plot or area. Even in an open world, it is okay to place that giant that can kill you at level 1 in the same cave your first quest is in. Why? Because you don't have control over the world. That giant was just minding its own business. If you were stupid enough to try and fight it at level 1, when several bones and skulls and what-not were sitting right next to it, that's your fault.
Now, the problem with this sort of gameplay is that people take this sort of thing as a challenge. You can't NOT kill the giant at level 1. No matter how obvious you make the "this guy will kill you pretty fast" sign, someone will tell you "omg, dis game sux! u cant even kill da gaint!" I'm sure you can't at level 1. That's the point. There are some challenges that you cannot beat at that moment. But, that's how the world works. That's the challenge. The challenge isn't making the giant more powerful as you get more powerful. As long as there's a way back, you don't have to be able to kill it at that moment.
I get it, though. It punishes grinding. But, really, grinding is its own punishment. I'd rather not have the same difficulty no matter what level I am. If I'm level 50 in a level 30 zone, I just wasted like 4 hours of my life making this stupidly easy. Go me! I'm SOOOO COOOOOL!