I have searched Google and Wikipedia, but I have been unable to find exactly what Linux is missing to reach POSIX compatibility.
Maybe ask on the LKML? 
According to this page, which is very old, the answer is $15,000.
lemme go out on a limb here…
i’ll say copyright is what is missing.
This page has been pulled from the MS FUD… at least the original arguments
Linux is 99.5% POSIX compliant and only a few things are not, mostly since Linux tends to be biased more towards GNU than POSIX and there have been on occasions disagreements between how POSIX dictates certain things and how Linux developers see it… re: according to POSIX it’s perfectly normal for a file system to lose your data and POSIX itself does not say much about how a file system should be implemented to guarantee user data consistency. Linux file system developers think this part of POSIX is braindead garbage and tend to not follow it and try to assure and implement better methods for data consistency while at the same time following other parts of POSIX about file system implementation, which are not so braindead
In [1]:
4.3 How source code compatible is Debian with other Unix systems?
…
Linux is intended to adhere to POSIX.1, but the POSIX standards cost real money and the POSIX.1 (and FIPS 151-2) certification is quite expensive; this made it more difficult for the Linux developers to work on complete POSIX conformance
…