Things which should be straightforward:

 * The following options should be supported but aren't:

	--expand-tabs
	--initial-tab
	--ignore-case
	--ignore-all-space
	--ignore-space-change
	--sdiff-merge-assist

 * Optimise the case of the --brief output style, where a full-blown diff
   isn't necessary but we currently do it anyway.  Similarly, if no diff
   implies no output (which it does for some output styles) we could
   avoid a full diff sometimes.

 * We currently aren't careful about noticing the difference between a
   file which has a return/new line on the last line and one which
   doesn't.  Admittedly this distinction has never made a difference to
   any diffing I've done, but if we're going try to be compliant...


Things which need a bit more work:

 * Implement a more efficient diff algorithm.

 * Support diffing of binary files.  Mostly this just requires being
   more careful than we currently are.


Things which would be nice but may/will require a lot of work:

 * Implement a "regexp" module so we can handle more options.

 * Output file dates on context/unified diffs.

 * Directory diffs.