BSD Syscalls include all of the functions present in libSystem.dylib, which are internally implemented using a system call. FreeBSD Syscalls.
HFS+ is case insensitive -> all calls with file paths must be checked for the correct case before passing to Linux.
kqueue API is not implemented by the Linux kernel. It can be emulated via libkqueue.
Apps with copy protection access system calls directly. This normally won't work out on Linux. PTRACE_SYSEMU could work for something.