QEMU user space emulation has the following notable features:

System call translation:
QEMU includes a generic system call translator. This means that the parameters of the system calls can be converted to fix endianness and 32/64-bit mismatches between hosts and targets. IOCTLs can be converted too.
POSIX signal handling:

QEMU can redirect to the running program all signals coming from the host (such as SIGALRM), as well as synthesize signals from virtual CPU exceptions (for example SIGFPE when the program executes a division by zero).

QEMU relies on the host kernel to emulate most signal system calls, for example to emulate the signal mask. On Linux, QEMU supports both normal and real-time signals.

On Linux, QEMU can emulate the clone syscall and create a real host thread (with a separate virtual CPU) for each emulated thread. Note that not all targets currently emulate atomic operations correctly. x86 and ARM use a global lock in order to preserve their semantics.

QEMU was conceived so that ultimately it can emulate itself. Although it is not very useful, it is an important test to show the power of the emulator.