(Good) Philosophical arguments are essentially back and forth exchanges of long, coherent and self-sufficient arguments. Thus, I think the character limit on comments is not enough for proper responses to answers and this feature should be allowed if one is to explain exactly how the answer has failed to address the question/is wrong/is incoherent/etc.
Furthermore, since debates should be visible and since a response to an answer may be just as interesting as the answer itself people should be encouraged to view them as one, perhaps we could also get larger fonts and/or bolder typefaces for comments. Perhaps up-votes on comments could enhance visibility accordingly.
I feel that these two features will be especially forthcoming in a philosophy board.