My take, in planning apotential 4e campaign:
In Heroic, the PCs change history;
In Paragon, the PCs make history;
In Epic, the PCs break history.
Which is to say, in my thinking, in Heroic, the heroes are just that, heroes and recognized great woman and men (or whatever), the ones who everyone recognizes that if they hadn't been there, things would be very different (i.e., much worse); battles would have been lost that were won, nations that would have fallen instead were saved, and so forth (real-world examples might include Winston Churchill, or Robert E. Lee, or Lech Walesa). Paragon heroes leave a lasting, important mark on the world: a major disaster or crisis was staved off; a nation that becomes a great empire would never have bene strong, or have existed at all without these folks; the big evil empire would have crushed the resistance, stuff like that. History would be hard to imagine without them (Picture such people as Charlemagne, or Julius Caesar, or Tokugawa). Epic tier is almost beyond analogy, people who simply introduce a complete change of historical currents, the end of an era - Napoleon, or Genghis Khan, maybe - but really, in a fantasy world, this is nothing less than altering the fabric of reality.
The reason I'm pondering this kind of stuff at all is that I'm simmering a potential 4e campaign, and I want to have an idea where it's all going from start to finish. While obviously I can't plan too much detail before I even know who the heroes are, I want to have broad schemes in place, partly for my own planning, and partly so I can have common threads weaving throughout, as well as useful prophecies and similar material.
Anyway, back on topic: I'd be interested in seeing something like a GM3 for epic tier and such. I just read Sly Flourish's Running Epic Tier Games, and it's a very good start. I also can't help but notice just how high the new online 'monster builder' goes, so you can scale everything up and down at will.