[chemotaxis]

It's a completely different thing. Single-board computer versus a microcontroller. It might not matter for some applications, but it's a major tradeoff.

But the only value of Arduino is the community (and the compile-time layer of syntactic sugar, if you like it). Otherwise, it's just an expensive breakout board for a cheap chip you can buy from Mouser or DigiKey. If you know how to solder, you don't really need the board in the first place.

[KalMann]

Aren't you forgetting about the software that makes it so easy and straightforward for newcomers to flash programs and experiment the microcontroller?

[chemotaxis]

First, the software is available whether you buy the board or not.

Second, there's no real difficulty barrier, not anymore. There are point-and-click tools, free integrated IDEs, cheap programming dongles, etc. There are more tutorials for Arduino than the underlying chip, and I'm not saying that doesn't matter - but it boils down to the community, not the hardware.