It was the first x86 chip with an instruction cache. This is significant not only because of the speed increase, but because the 486 was the first chip that unmodified 8086 software started having compatibility issues with. Frequently, the only solution was to disable the 8k internal cache, and I remember downloading several programs from a local BBS which managed to do just that in various ways.
The instruction cache is what makes a 40Mhz 386 (with a 8Mhz turbo toggle) the king of oldskool gaming. It just doesn't get any better compatibility-wise.