Thoughts...
          From Anne.
             Dog Quest                 |   2642 Bernardo Avenue, Escondido, California,  92029  |                     760-489-0537             |           DogQuest@cox.net         
I've been wanting to do this for awhile - compile a cheatsheet of sorts as a reminder list for the rule differences in USDAA.  In a way, USDAA can be confusing (especially to judge) because the rules for refusals and wrong courses for example are applied differently to the different levels. I'm always hesitant to write too much about rules in agility because the rules change, it's a growing and developing sport, and you have to be sure that what you write is still current :)  A word to the wise, try to be cautious about reading too much outside of the current rule book for the organizations, otherwise you might be reading some old stuff!
This page was last updated on: June 15, 2011
Anne's reminder notes about USDAA rules

Except for pairs and snooker, your run must be clean, without faults, to qualify.
Placement is still awarded (1-4th) even if you did not qualify.
A missed contact, wrong course, knocked bar, leaving the table before "go", not completing an obstacle, any fraction of a second over standard course time, are all faulted and will cause you not to qualify.
USDAA faults all missed up and down contacts.
You must run your dog naked. (without a collar on. )
The table performance is always a down. 
You can "correct" the weaves rather then having to start again at the first pole.

Equipment
Championship jump heights - Mini = 12" and 16", Open =  22" and 26"
A-Frame is set steeper 5' 9"-10" (versus 5' 6") for Open dogs
The dogwalk and teeter contacts (yellow zones) are shorter then AKC
Spread Jump: mini (ascending) 10"-12" spread, Open (parallel) 12"-15"
Extended Spread Jump: always ascending,  Mini = 12" Open =  15"-24"
Long Jump Spread = 20", 36", 48" and 60"
The table height for Championship 12" = 12", 16"=16", 22" & 26" = 24"
Performance jump heights - Mini 8" and 12", Open 16" and 22"
No spread jumps for performance dogs
The A-Frame is set at 5' 6" for all performance dogs.

Novice
Refusals are not faulted
Wrong courses are faulted
The weaves will be a full set of 10-12 poles.
The table count is consecutive.
4 Paw Rule

Advanced
No 4 Paw Rule
The table count is cumulative
Weaves - improper entry is not faulted but missing a pole after a correct entry is faulted
Refusal are only faulted on the approach to the contacts

Masters
No 4 Paw rule
The table count is cumulative
Refusals a faulted very similarly to AKC except the interpretation of a refusal is slightly different. A dog can turn back on their path as they approach the correct obstacle and will not be faulted as long as they make progress to that obstacle. (A spinning top is ok).

A wrong course is whistled signaling an "E" for Elimination.  Usually you can continue and finish the course.

Pairs = Is scored "time plus faults". This means that they will take the total time for the pair to run the full course.  If either dog has faults it will be added to the team's time.  This score will be compared with the standard course time to determine if you qualified as a team. A good team, with moderate speed can usually qualify even with one standard fault.  A fast team can usually still qualify with 2 standard faults but it is very rare to be able to qualify with 3 standard faults. Except in Novice, a wrong course will cause a team not to qualify.