Part of the problem is writing the exam questions in the first place.  Others have noted that, when designing a grading rubric, you should identify what the key skills in the problem are.  This seems backwards to me.  *First*, identify the key skill that you want to test, and *then* write the exam questions which hit those skills.

Once the exam has been written, I would very much recommend that you make the rubric as simple as possible, given that you want consistent grading across a (possibly large) group of TAs.  I typically grade on a 3 points scale:

* [3] The answer is nearly perfect.
* [2] The answer contains errors that are mechanical in nature (e.g. missing signs, incorrect computations, etc), but not conceptual.  The mechanical errors are minor or are not central to the skill(s) being tested by the question.
* [1] There are serious mechanical errors and/or conceptual errors, but *something* correct or relevant has been written on the page.
* [0] The answer is essentially ungradable (it is blank, or nonsensical, or whatever).

I will note that my [2] and [1] are essentially the 50% category in [this answer](https://matheducators.stackexchange.com/a/25854/).  I think that it is worthwhile to distinguish between "dumb" arithmetic mistakes and more fundamental conceptual errors.  That being said, my scheme is essentially the same idea—simple and quick to implement.

Experience has shown me that students really don't *like* this system.  The students I work with are used to a grading scale in which an A is anything over 90%, a B is anything over 80%, and so on.  Thus when they get 2 points out of 3 (67%), they feel like they are failing (since 67% is a D).  However, my feeling is that [2] represents, roughly speaking, B or C level response, while a [1] represents a D or low C.  This is something which has to be thought about when assigning letter grades—either add more "free" points into the course elsewhere, or grade on a different percentage scale, or accept that more students are going to fail.

If you are really stuck on a 90/80/70% scale, then remap [3] to 5 points, [2] to 4 points, [1] to 3 points, and [0] to 0 points.

If you want to weight different questions differently, continue to grade them on a 3 point scale, but weight them differently (easy-peasy).  Students will be most happy if all of the questions are worth multiples of 3 points (because they don't really want to think about weighting).

In any event, the overall goal is to construct a grading scheme which is **fast** (you don't want your graders to have to spend a lot of time on things), and **consistent** (different graders should score a given response in the same way).  Creating lots of deductions or opportunities for partial credit makes grading slower, hence I would tend to avoid it.  Consistency is also easier to attain if there are fewer categories.