I haven't used "Mavericks" that much yet, but one thing that caught my eye early on was a change in df(1):

$ df -h /
Filesystem   Size   Used  Avail Capacity iused    ifree %iused  Mounted on
/dev/disk1  118Gi   14Gi  104Gi    12% 3626080 27340805   12%   /

Wtf? Why are they displaying inode allocation by default? Well, in their manpage they state:

 -i      Include statistics on the number of free inodes. This option is now the default
         to conform to Version 3 of the Single UNIX Specification (``SUSv3'')
         Use -P to suppress this output.

Ah, SUSv3. That was released in 2002 - of course MacOS 10.9 has to catch up on portability here, sure¹ :-)

Let's look at SUSv3 (aka "IEEE Std 1003.1-2001") then:

   The df utility shall write the amount of available space [XSI] > and file slots <

And this XSI stands for "Extension":

The functionality described is an XSI extension. Functionality marked XSI is also an
extension to the ISO C standard. Application writers may confidently make use of an
extension on all systems supporting the X/Open System Interfaces Extension.

So, it's not so much a conformity issue but a mere choice to include the ouput of inode usage in the newest iteration of MacOS X. While it's still not clear why they changed it (and left -t a no-op), let's look at other Unix versions:

$ uname -srv; df -h /
SunOS 5.10 Generic_144500-19
Filesystem             size   used  avail capacity  Mounted on
/dev/md/dsk/d1         7.9G   3.0G   4.8G    39%    /

$ uname -srv; df -k /
AIX 3 5
Filesystem    1024-blocks      Free %Used    Iused %Iused Mounted on
/dev/hd4           262144    175244   34%     3919    10% /

To restore the old behaviour in MacOS, use -P:

$ df -Ph /
Filesystem   Size   Used  Avail Capacity  Mounted on
/dev/disk1  118Gi   14Gi  104Gi    12%    /

¹Overlooking the fact that their manpage still reads "May 8, 1995" at the end...