Here are explanations of how I understand the different grades, when marking philosophy papers.
A
This is a truly outstanding paper. It is clearly written, well-argued, and original. A paper that just gives a straightforward or "obvious" response to some philosophical or interpretative problem would not merit an A, even if it is clear. An A paper does something extra--but not at the cost of a clear treatment of the problem. If you receive an A on a paper you have reason to feel extremely proud of your work.
A-
This is a really good paper, one that operates at an advanced level. It is clearly written. (If there are any significant problems with the writing or the organization of the paper, then it won't merit an A-. This is because good clear writing and organization are not separable from good philosophical thinking.) The paper may have a couple of minor mistakes or confusions, or it may fail to unpack some of its arguments sufficiently. It may have an original argument or interpretation, but if so, that will be offset by some other flaw. For example, in a longer paper, perhaps there is too little philosophical back-and-forth (considering objections and challenges, and responding to them). Or perhaps it is not as engaged with the texts as it should be.
B+
This is a well-written paper with nothing terribly wrong. The writing may have some small problems, or it may be flawless. The paper may make some mistakes or have some ambiguities that have to be sorted out, but overall it will be a good paper. It will show more promise or originality than a B paper, but nothing will make it stand out like an A- paper, or it won't be operating at as advanced a level as an A- paper. (Or perhaps the paper would stand out if some of its ideas were properly developed, but as it stands they aren't.)
B
This is a solid paper, with some notable mistakes or obscurities, but no serious misunderstandings. The writing may not be super-clear. To earn a B, the paper needs to make it clear why the problem addressed in the paper is a problem, and offer some response to it. (It may be a straightforward or unoriginal response; it may not be a decisive response; the paper may even end by showing that a certain response doesn't work. But the paper must put forward or examine some response to the problem.) A B paper does not seriously misrepresent the views of other philosophers.
B-
There are starting to be some serious problems. Perhaps the writing is really unclear or the paper is poorly organized. Or perhaps there are straightforward mistakes and misunderstandings about what the problem is, or about what other philosophers say. Or perhaps the paper presents the problem correctly, but doesn't really address it. Still, there is an effort. The author has some understanding of the problem and of the relevant texts. She does offer some argument. A paper with no argument won't merit a B-.
C+
There are more serious problems. Either the writing is really hard to get through; or the paper has no discernable structure; or the author doesn't understand the text or the positions she is discussing; or the paper doesn't really attempt to offer any argument.
Papers with more problems will earn grades of C or below.
It is difficult to give a general gloss on those grades since the problems that beset these papers are quite varied.

In my experiences teaching at several universities, fewer than 10% of students are able to write above the B- level in their first attempts at philosophical writing. (For what it's worth, some of my first philosophy papers were B-s, too.) Don't let this discourage you. Doing and writing philosophy are hard. But they are skills you can learn. People who make a serious effort to learn them are often writing B+ or A- papers by the end of term.