I assume that before asking the question, everyone read the latest exam reference guide found here:
http://www.arubanetworks.com/pdf/education/ERG_ACMX.pdf
Having authored many of our MX exams, and having proctored many, many exams around the globe, I can assure you that I see patterns between failures and passes.
For example, one sure way to fail is to execute "write erase all" on the controllers at the end of the exam. Yes, I have actually seen that several times. :)
Also, let me stress that every task and skill that we require in the exam is covered in our courseware. We highly recommend MBC, or the IAW/SWDI equivalent, AirWave and Advanced Troubleshooting. The primary difference between the courseware and the exam is that we are testing to see if you can apply the knowledge in a larger scale environment. Hence you can expect to see more than two controllers and APs in the exam unlike in the classroom. It seems to me that many candidates have not attended the entire recommended sequence of classes and therefore approach the exam with a less than complete exposure to the required skills. There are a couple of exam tasks I can specifically point to that demonstrate this to me. If you are not going to attend the classes, then be sure you pick up the equivalent skills from somewhere.
Additionally, we are testing if you can readily recall that knowledge without needing to rely upon reference materials. If you need to review the manual often, you will not have time to complete the exam within the alloted 8-hours. Therefore, the second recommendation is familiarity with the kind of tasks outlined in the Exam Reference Guide.
Within the exam, we do ask you to do many kinds of common tasks. But sometimes the candidate does not read the requirements carefully, or doesn't take the time to implement them as requested. I will say the Firewall policies are one of the more common places where we see this. It is easy to use wizards, but rarely is the baseline template sufficient in a real deployment, is it?
Thirdly, we test the most common feature and deployment scenarios that an expert level individual is likely to encounter in a real deployment. Again, we list those within the ERG. An example here is as basic as getting the correct IP settings. You must understand how to deploy controllers in L2/L3 and firewalled environments. These are seen in all networks and are discussed in the courses. Hint, hint.
That's my 10,000 foot recommendation and expression of the exam philosophy. I will post more later if I think of more things to share without spilling exam specifics.
Kevin