Convention over Configuration
Stick with what is done by consensus, that way everyone understands.
Don't Repeat Yourself
Bugs per lines of code tends to be constant. Reduce the number of lines of code you write and the bug could will go down.
Pervasive MVC
MVC triads exist everywhere in a RIA, so express them as such. Nest them and reuse them.
Use Data Binding
Imperative data management leads to more lines of code (see DRY). Declarative data management increases the readability of code.
Write Good Tests
You never know what's going to break, or where. So keep looking.
Automate Tasks
Repetitive and scriptable tasks can be done by an intern or a script. If you are doing repetitive tasks, and are not a script, what are you then?
Theses tenets are based off of The Rails Tenets with some alteration in tenet 3 and 4 to make it applicable to RIA development over development across a full web stack.