They provide you with a long, muddled list of guidelines to follow. They also request that you try to complete the markup within 30-45 minutes (this time limit seems to be their main point of contention but isn't something that's stressed heavily in the instructions, and there's no indication that you might be timed during your markup).
Ultimately, I wasn't offered a position. I have a Bachelor's degree, so maybe I wasn't what they were looking for at the time. Smarthinking brags that 90% of their tutors have Master's or P.h.D's.
Like others on this site have said, completing a paper review in half an hour is stressful and somewhat unrealistic. I know I didn't finish my sample markup in 30 minutes (possibly the reason I wasn't offered the job). You're reading and re-reading a student's paper, scanning for all types of mistakes, and writing comments meant to teach the student WHY something is incorrect rather than just calling attention to something that is incorrect (it's a solid teaching strategy but not something that's easily done under a time limit).
In my opinion, this company pushes quantity over quality. If I were a student and I paid for this type of service, I wouldn't want my paper reviewed by this company. They can't look at your work in its entirety and deliver thorough writing advice - it's just not possible.
I researched Smarthinking, and I would say don't apply for this job if you are considering this as your main source of income. The pay is minimum wage or slightly above minimum wage. If you have a college degree, like they are asking for, then you're most likely overqualified for this type of work anyway.