In my department and my TA position, the common advice is to go with your gut. Is this an A speech? Is this a B+ speech? Subjectivity is everywhere, but we must be consistent and fair among all students.
I teach public speaking, and am often confronted with challenges from students on the grades I assess. The best thing I can do to inoculate my methods from these attacks is to be very thorough and clear about my grading practices, and to apply those practices fairly across students. It is a problem I believe most instructors in the humanities face in their teaching, but as a younger GTA I find myself to be an easy target for students who understand the challenges of grading a course like this.
My brother teaches geology in his assistantship at an institution similar to Virginia Tech. He was discussing with me a grading dispute with a student the other day. The student argued that they had put much effort into the task, but my brother was resilient. He held to the facts–the student’s work did not ring true with fact, the literal rock-hard truth.
What constitutes a good speech is not so clearly defined, and that is a burden that those in my discipline will bear while those in the sciences can easily escape.