When looking at measurable objectives for writing, check the spelling. As well as words that are incorrectly spelled, make sure the right homophones, such as "to" or "too" are correct. Failure to pick up these sorts of mistakes indicates that the editing was not done properly and that can be assessed accordingly.
Style issues -- such as run-on sentences or incorrect punctuation -- can also be addressed. Another measurable indication of good writing is how well the student followed the rules of grammar. Common mistakes include errors such as subject-verb agreement or sentences without a proper subject or predicate.
Another way to measure writing is to assess how well the student followed the instructions of the assignment. If, for instance, you asked for a 3,000-word essay and you received one that was 1,000 words, you can be sure the person wasn't paying attention to the assignment. Similarly, if the student strays off topic -- or totally misses the point -- it is a measurable error.
As well as how well a student uses language, you can also measure higher level thinking skills. Juxtapose the essay with Bloom's Taxonomy and see how many times the student calls on the higher level thinking skills of analysis, synthesis and evaluation. If the assignment called for this and the piece handed in only addressed the knowledge, understanding and application levels, you can gauge the shortfall.
Another measurement tool is tone and voice. In an academic essay, for example, the tone needs to be detached and researched. If the student submits an emotional piece based on opinion rather than fact, you can calculate how far it missed the target. Similarly, writing in the passive voice -- unless specified in the assignment -- determines whether the piece is easy to read and understand.
You can evaluate whether or not the writing is logical. No matter how eloquent the words, if the writer confuses the reader by jumping from point to point without a proper transition, the reader is lost. Similarly, presenting the argument by starting with the supporting statements and then presenting the thesis statement indicates poor writing.