Most teachers I know (including myself for the first 4 years in the profession) take off a half point for missing labels and/or incorrect negative signs.  Why is this tradition ingrained in our heads?

My guess: Our teachers did it to us, so we do it to our students, too.  Dan Lortie calls this the "apprenticeship of observation."  In a nutshell, Lortie suggests that we teach in ways we were taught in school.  This is why standards-based grading is so difficult for the majority of secondary teachers to comprehend and adapt for themselves.  Points were (and continue to be...except for the innovative folks listed here) the classroom currency of choice in the overwhelming majority of U.S. public school classrooms.  If it ain't broke, don't fix it, right?

Uh...our traditional grading systems are broken

Take this test question as an example.

This student incorrectly answered question #4.  Are the negative signs a big deal?  If the question was worth 5 points, is a half point deduction reasonable?
A closer look at the problem indicates the student reflected the points over the line y = 0 rather than x = 0.  Most likely the student either misread the problem or does not know the difference between these two lines.  My observations tells me the latter is the more likely of the two for this particular student.  The negative sign here do not show a careless error, but instead indicates a misconception.  Does the student know he/she has this misconception or was it just one of those negative sign errors Mr. Townsley takes a half point off for...because he always does that?

Traditional system: Student takes the half point hit.

Standards-based grading: Teacher is forced to analyze what the student might have done wrong rather than blindly taking off points.  Student's score reflects the level of understanding, not a token value based on pre-determined deductions for labels and negative signs.  In this case, negative signs matter.  The student is made aware of this through feedback and the learning target score.

Have you switched to standards-based grading yet?  

Why not?