It's hard to describe other than, the new model m is just better than previous classic and endura pro.

I find them more sturdy, almost no key wiggle (as opposed to having some before), the lettering on the keys are clearer (although that I didn't notice until someone pointed it out and didn't care after, it was something people complained about), the enclosure doesn't creek if you try to flex it etc.

My only complaint is that I had to put white tape on the LED indicators to diffuse the blue LEDs because they're distractingly bright at night otherwise.