OP -- To be fair - the 2600 was the big breakthrough --
custom sound and graphics chips -- on top of a CPU, at the same time everyone else was releasing "just CPU" systems. However,
The 800 is a very solid advancement of this, and brings some new technologies that were quite inspired:
- SIO, early form of USB (12V power over the cable, daisy chaining, smart devices that can identify themselves and load firmware)
- Proper expansion busses
- 3 or more Custom chips instead of 2 (hey, there's a lot of hardware in the A8), depending on how you count
- Fully FCC shielding compliant (Hello Apple II)
- Many excellent engineered Atari peripherals
Beyond those, a lot of it was iteration : more colors, more sprites, (much much) better sound, etc. add on ram, keyboard, and more joystick ports..
It's worth noting though the 800 is only 24 months newer.. They actually added quite a lot in that time to make it a more powerful system. The 800 could definitely reproduce arcade games and other games much better than the 2600.