Things I learned when my T.A. was away for a week

This is an intriguing observation. Can you go into more detail as to what made the present homework set difficult to grade and what you would change if you had the chance?

Humbling, isn't it? I seldom give myself an "A" when grading my assignments.

I've found that preparing a rubric that lists each result desired goes a long ways towards spotting problems that are difficult to grade. Still, the proof is in the pudding, and it's useful to revise problems that don't perform the way you'd like.

the guardian saint of TAs (though seemingly not the most effective saint out there) will bless you - both for doing the grading and for writing easy to grade problems.
That said, working with Bag of Tricks as a TA has always a lot of fun, which is actually more important than quick grading.

What's the book (ARM) you are referring to?

I think thinking about what they will be like to do is indeed much more important than how easy they are to grade.

Grading is just a way to tell the students (and a poor way, for that matter) how they did.

Both are important. If the results are difficult and tedious to grade, it reduces the amount of feedback I give to the students. If the grading is relatively quick, I have time to make comments to help students find the source of their errors or affirm them in their solid performance.

As E Bogue indicates, the advantage of making the homeworks easy to grade is that the grader (i.e., me, in this case) can quickly spot mistakes. Then when the students gets back the graded papers, they can see what they went wrong.

The current hwk asked the students to fit some models and do some comparisons. Basically I'm trying to give them practice fitting and graphing and understanding the results. But how do you know things went wrong? For example, you know this when you graph your fitted model and it isn't anywhere near the data. So this is something that I should've explicitly asked the students to do as part of the homework assignment.