The quality of the documentation is phenomenal. Very competent people clearly put serious effort into making it great.
Another stand out is the Programmable I/O feature. Basically, you have eight separate, programmable state machines that can manipulate I/O pins and do DMA. This lets you offload bit banging from the main cores. Details are in Chapter 3 of the C API docs: https://datasheets.raspberrypi.org/pico/raspberry-pi-pico-c-...
I'm a total amateur when it comes to embedded development, so I have no idea how the RP2040 sits in the competitive landscape.