Case in point, my current scenario. The heroes are stranded on a mysterious island populated mostly by harpies and sea-devils, monsters which in previous editions of D&D would have been beyond the capabilities of a party of three 1st-level characters. Now in some other system, HERO System, for instance, there is no "these monsters are power X, these monsters are power X+1, these monsters are power X+2," etc. — you just build the baddies with the stats you want them to have, and go. That was one of the things I used to love the most about HERO: I wasn't constrained by the system. If I wanted the characters to go up against harpies and sea-devils in the first adventure, well dangit, that's what they did.
Well, so far, the same is true of 4E, and that's earned it major points with me. System-as-toolkit is my thing. And it is remarkably easy. Want a 3rd level sahuagin guard? Take the base one (6th level), and click the "level down" button three times. Then give it a quick double-check to make sure there are no wonky bits, and it's done. Or, alternatively, you say you want a 2nd-level sahuagin skirmisher? Build a 2nd-level skirmisher, snag a "racial theme power" (in this case, blood frenzy), and you're done. Quite amazing, really.
But the real neat thing is the simplicity of "re-skinning." There's an encounter in this scenario which I lifted whole cloth from an example in the MM ... except the one in the MM was populated by hobgoblins, not sahuagin. So what did I do? Took the hobgoblin stat block, pulled out the hobgoblin "racial theme power," and slapped the blood frenzy on them instead. Voila! Hobgoblins instantly transformed into sea-devils, and it took about a minute and a half.
This little trick is hardly new to 4E, of course. "The Dragonpriest," a recurring feature of my 3.x Greyhawk game, started out as a creature that looked like a dragonborn (back before they were as common as dirt), but mechanically was a troll. But 4E, I must say, does make it easier. Like with my former hobgoblins listed above, as long as you tweak them a little to keep the thematic elements that make Creature X actually feel different from Creature Y, most of the numbers can be whatever they need to be to make the game go well. Sahuagin don't use phalanx tactics, they go into a blood frenzy. But does the fact that this particular sahuagin has the same AC as he would if he was a hobgoblin instead really make a difference? No.
This gives the GM a lot of flexibility, and in terms of adventure and/or worldbuilding, really opens the doors for the system to be used in ways other than the default. My current game isn't going to have goblins, for instance. They just aren't there! It's all "adventures of Sinbad" / The Odyssey, not Lord of the Rings. But if I wanted to, I could probably take a half-dozen already-published, goblin-based encounters, put a new skin on them, and drop them right in without difficulty. While it takes a little more effort to put the game into a context like this instead of just setting it in "D&Dland," I think the results will be worth it.