Okay, but even with the dragon example, how do our PCs have a barometer on when they're tough enough? I mean, by their standards they're much tougher at 8th level than 1st, but that may not be enough to face the dragon. 16th level? NPCs would have to be keeping tabs on the PCs in the form of "well, if you faced the Tomb of Eternal Slumber and escaped alive, you're tough enough to take on the dragon" or the like.By keeping as many detail realistic you get the benifit of how people of different experience interact in real-life. For example the PCs wouldn't go dragon slaying off the bat they may assisst the local noble in clearing out bandits and orcs from his demense. Then later get a job guarding a scholar in exploring a ruins. Then get to the point where they have enough contacts and experience to start perusing their own leads in more dangerous areas.
That still seems rather metagamey, or calling for an Elminster-like or Gandalf-like mentor or all-knowing sage, which aren't exactly welcome in many games, or make the PCs dependent on outside help. If the local farmers know about how dangerous the Tomb of Eternal Slumber is in relation to the dragon, there's suspension of disbelief issues.
Maybe this stuff is resolvable via the skill system - Knowledge (Local) giving you an idea of the creature CRs and dungeon CRs in the vicinity. That might make such skills a bit of a must, though - overinflating their importance in the game.