There's also way better input and output support.
I recently did a project with ucLinux on an LPC1788 (Cortex-M3). Getting the USB host stack is always a pain in the ass, but it was dirt simple on the Linux port.
Same for LCD framebuffer, GPIO, touchscreen (unique animals every time), I2C, SPI, etc etc etc. And I can pull anything I want off the kernel tree and not have to spend a week getting it working on another OS.
One good reason for using Linux instead of any of the RTOS is better networking support. I've used VxWorks, RTEMS, Nucleus, FreeRTOS, eCos, etc and their networking stacks are all pretty slow and crappy.