The 286 added protected mode, though not as nice as 386+. You could access up to 16MB of memory in 80286 Protected Mode. oh, baby!It was worse than that; the 286 provided no mechanism to switch out of protected mode. Windows-286 was a dog because it depended on so many DOS interrupt routines which had to be executed in real mode (since the drivers had not been written as native VXDs yet). The only reason this was possible at all was because a facility was provided to reset the CPU through the keyboard controller. Every time the software needed to switch the CPU into real mode, it had to be completely reset through the keyboard controller. You can read all about the A20 mechanism here [win.tue.nl].
The 386 finally added the ability for the control program to switch back to real mode. With that it carried a bug that allowed the user to set a segment limit of 4GB while in protected mode and then quickly switch back to real mode, giving the user access to a 4GB address space in real mode (where only a 1MB address space should be available). Many games and demos circa 1992-1993 exploited this "Unreal-mode" feature like Ultima 7 and Zone 66, and were known to be the nastiest, most incompatible programs ever to exist, never getting along with any memory manager or multitasking operating system. If only game programmers had used something sane like DPMI back then!
IIRC the 486 added something related to cache, I forget if it was on-chip cacheYes, it added an 8KB on-chip instruction cache - to the dismay of many legacy programs which used precisely timed tight loops for program timing.