PCs are special people by any measure. Even at first level, they're impressive specimins of their profession, and often qualitatively better than any ordinary man or woman. A first level fighter isn't at par with a green rookie; he's got the stamina, skill, and honed combat senses of a grizzled veteran, however young he may be. A first level priestess isn't just an ordinary cleric, but one of the few people on the isle capable of calling down instantaneous evidence of divine favor.
In terms of combat capability, PCs stand head and shoulders above their peers. As a general guideline, any normal foe of 5 or more levels fewer than the PC can be treated like a level+5 minion. Thus a level 3 human guard might be treated as a full-hit-point regular enemy against a group of 6th level adventurers, but against an 8th level PC they're treated as an 8th level minion. This represents the guard's ability to still be something of a threat, yet no real danger to an 8th level PC unless they come in large numbers.
Any normal foe of 10 or more levels fewer than the PC can be treated as a scene prop, unable to be any threat to a PC unless they come as an army. A paragon-level PC simply isn't going to be threatened by ordinary men and women, or common goblin spearmen. They're too skilled, too powerful, and altogether too Heroic to be fazed by a peasant mob. As they rise in level, more and more foes essentially become speedbumps to them, no meaningful challenge or danger to them unless overwhelmingly numerous. Even then, it can prove inadequate; an epic-level fighter is exactly the sort of hero who can single-handedly break armies of common soldiers hurled against him.
DMs are not compelled to follow these guidelines, and they may particularly skip them when in a group of mixed-level PCs. On the whole, however, they describe adequately how PCs might relate to a goblin swarm- lethally dangerous at low levels, a minor challenge at mid-Heroic, and mere set dressing at Paragon-plus.