[nomel]

I never really understood how GPIO is a killer feature with them. There are so many ways to get GPIO, from $5 USB dongles to any microcontroller/dev board that's ever exists. What's special about Raspberry Pi GPIO that I'm missing?

The only case I can think of is very heavy compute that relies on low latency GPIO related to that compute?

[robinsonb5]

The low latency is the reason why the PiStorm (Amiga CPU accelerator) project works so well on a Pi 2, 3 or 4. (Pi 5 is no longer suitable since the GPIO is now the other side of a PCI-E bus and thus suffers significantly higher latency than on previous models, despite being much faster in terms of throughput.)

[hypercube33]

In general the best part of the Pi is that it's so stable as a platform. Everyone has the same hardware and generally the same OS so guides online just work - it may be one of the if not the best Linux on the desktop experience I've used personally.

Along with that the gpio is there and ample so it's extremely easy to just start using it.

I do argue an esp 8266 or esp32 are better for a development microcontroller but you have to muck with cabling it up before you can even load a program on it which is a few more extra steps than a Pi

[regularfry]

If it's a funky esp board, possibly. The esp8266 and esp32 boards I've used all have usb sockets for programming.