1) Mild to severe depression
2) Acute boredom
3) Mood Swings
4) Randomly breaking out in maniacal laughter
5) Randomly breaking out into monologues/quotes/songs from the show, all memorized to the exact emotions, pauses, and punctuation.
6) Randomly breaking down in hysteric tears
7) Randomly moaning
8) Sudden urges to run to AvatarSpirit.net to check for any updates, even though you just checked 3 minutes ago.
9) Constantly on the Internet searching for any sign of the return of A:TLA
10) Having dreams about the return of A:TLA only to wake up and discover it was just a dream
11) Start to abandon your favored shipping and turn to the dark side
12) Snap out of it and start crying because you realized you almost lost hope
13) Worst case scenario is that fans curl up into the fetal position in a corner of their room and start rocking back and forth, chanting the theme song over and over.
If symptoms continue for an unbearable period of time (which could be over 2 weeks to X number of months depending on the person) the recommended remedy is to surround yourself with Avatar related stuff: make your own fan art, fanfictions, make a petition to Nickelodeon saying that you're fed up with the wait and then never send it, as hard as it is, try and find another show that is almost as good as Avatar and watch that, talk to friends about how great the show is, buy more merchandise, etc.
AWS strikes hardest in fangirls rather than fanguys for reasons not yet proven. Some say that fangirls' constitutions are too weak to deal with the stress and are prone to break down more quickly. My personal opinion is that fanguys are too self-conscious to let their fanguy-ness show.
My message to people suffering from AWS is to take this time to convert others into Avatards and brush up on your Avatar knowledge, but always remember that the show WILL come back; never loose hope!