In beer judging, there are style guidelines. They make it easier to determine what a beer should be like. With meads, it's a lot more about what the entry says! For instance if you say it's a strawberry melomel and there is no strawberry, then it'll get less points. If you say it's a dry mead and it's sweet, you get lower points. If you say it's still, but there are a few bubbles coming from the bottom of the glass ... well, you get the point!
Declaring the honey is also important. If the judge can't identify the honey, they assume something like a wild flower. So weird tasting honey needs to be identified to get points for it.
Defects are the same as any beverage! If it has some, they are a problem.
In the end, it's more about balance than anything else. If you made a sack mead and it tastes like rocket fuel, saying it's a sac will get you a point for your description, but the balance towards rocket fuel with not enough sweetness or too much acidity will make it score lower.
I took the mead exam last year. There were not many oppertunities for classes. Study the mead study guide and learn all of the honey flavors. Go out and find those honey's and get a chance to ferment and taste them. Read all of the fruit descriptions. Memorize those! They are big on the test. Know how to describe each of the honey and fruits in the study guide on the BJCP web site.
Go here for examples of what to expect.
http://www.bjcp.org/mead.php
I would highly recommend you judge in a few BJCP contests BEFORE taking the test. You can judge as a novice. If you are an experienced mead maker, tell the coordinator that and let him/her know you are studying for the mead judge exam. They may be nice and team you up on a mead table. You can also be a steward. Again, tell the coordinator that you are studying for the mead exam and ask to steward at a mead table. Listen to the judges. Sample with them and ask questions. Some judges will be the quiet types, but many are more than willing to discuss the mead after they have finished their score sheets. If there are novice judges, there will be a bit of discussion while filling the sheets out to help the new judge know what they are tasting/smelling. Listen in and feel free to ask questions also.
Find a local beer club and get them interested in mead! Many clubs are starting to offer study sessions. My wife and I are planning one for this fall for our club.