Here is a (possibly incomplete) list of missing features and limitations:

The debugger- and profiler-related system calls do not exist.
The old 6th edition seek() was implemented, instead of lseek().
The supplied TTY driver is bare-bones. It supports only one port, and most IOCTLs are not supported.
Inode numbers are only 16-bit, so filesystems are 32 Meg or less.
File dates are not in the standard format. Instead they look like those used by MS-DOS.
The 4.2BSD execve() was implemented. Additional flavors of exec() are supported by the library.
The format of the device driver switch table is unlike that of the 7th Edition.
The necessary semaphores and locking mechanisms to implement reentrant disk I/O are not there. This would make it harder to implement interrupt-driven disk I/O without busy-waiting.

A Description of this Release:

Here is a list of the files supplied, and a brief description of each: (Includes things like errno.h).

intro:	What you are reading
config.h:	Setup parameters, such as table sizes, and the device driver switch table.
unix.h:	All strcuture declarations, typedefs and defines.
extern.h:	Declarations of all global variables and tables.
data.c:	Dummy to source extern.h and devine globals.
dispatch.c:	System call dispatch table.
scall1.c:	System calls, mostly file-related.
scall2.c:	Rest of system calls.
filesys.c:	Routines for managing file system.
process.c:	Routines for process management and context switching. Somewhat machine-dependent.
devio.c:	Generic I/O routines, including queue routines.
devtty.c:	Simple TTY driver, slightly-machine dependent.
devwd.c:	Hard disk driver. Very machine-dependent.
devflop.c:	Floppy disk driver. Very machine-dependent.
devmisc.c:	Simple device drivers, such as /dev/mem.
machdep.c:	Machine-dependent code, especially real-time-clock and interrupt handling code.
extras.c:	Procedures missing from the Q/C compiler's library.
filler.mac:	Dummy to make linker load UZI at correct address.
makeunix.sub:	CP/M SUBMIT file to compile everything.
loadunix.sub:	CP/M SUBMIT file to load everything.