Tony Vargas
Legend
The 4e 'treadmill' was, well, it was a very realistic illusion.
4e set out to extend the 'sweet spot' at which D&D was most playable in past eds, and it succeeded. 4e played very well at levels 1-30 vs similar-level enemies.
If you always used same-level enemies (which, even the encounter guidelines didn't tell you to do), you would, if you kept up on everything and were prettymuch 'optimized,' hit about the same and get hit about the same regardless of what that identical level was.
Now, think about that, the system that rates monster challenges, actually delivers the same challenge when you use a same level monsters. This is only shocking and disconcerting to people who have never encountered a system that worked before.
Where the treadmill illlusion is shattered is when you go back and fight something you've taken on a few levels ago. For instance, if you first encounter a 12th level monster when you're 9th level, it seems pretty nasty, it doesn't seem to miss you often, and you're having trouble hitting it and so forth. If you encounter the same kind of monster again at 11th and 15th, you'll notice it's gotten a lot squishier - that the 'treadmill' was an illusion. (As a DM, you can take it a lot further than that - you can take a low level solo and 'demote' it to a higher level elite, standard, or even, 17 levels higher, a minion at the same exp value...)
5e, BTW, still has a treadmill illusion, it's just 2-6 over twenty levels instead of 1-30 over 30, and you can collect magic items and boost stats over and above it. So it's a slower treadmill and that bar on the front that you hold onto has a tendency to break off. ;P

If you always used same-level enemies (which, even the encounter guidelines didn't tell you to do), you would, if you kept up on everything and were prettymuch 'optimized,' hit about the same and get hit about the same regardless of what that identical level was.
Now, think about that, the system that rates monster challenges, actually delivers the same challenge when you use a same level monsters. This is only shocking and disconcerting to people who have never encountered a system that worked before.
Where the treadmill illlusion is shattered is when you go back and fight something you've taken on a few levels ago. For instance, if you first encounter a 12th level monster when you're 9th level, it seems pretty nasty, it doesn't seem to miss you often, and you're having trouble hitting it and so forth. If you encounter the same kind of monster again at 11th and 15th, you'll notice it's gotten a lot squishier - that the 'treadmill' was an illusion. (As a DM, you can take it a lot further than that - you can take a low level solo and 'demote' it to a higher level elite, standard, or even, 17 levels higher, a minion at the same exp value...)
5e, BTW, still has a treadmill illusion, it's just 2-6 over twenty levels instead of 1-30 over 30, and you can collect magic items and boost stats over and above it. So it's a slower treadmill and that bar on the front that you hold onto has a tendency to break off. ;P