Good NETiquette is just like regular etiquette - consideration for other people. No one should dominate discussions or post needless messages that waste bandwidth and time. Users should use subject lines appropriately for all e-mail and respect topics in a discussion thread. Here is a good web site devoted to some simple rules of netiquette. That's when you learn to use a hammer, so everything suddenly becomes a nail.
Threaded discussions are effective when used sparingly and with a clear focus.
This course, for example, includes a discussion topic for each module, but course facilitators are encouraged to use or not use them as the needs of the group dictate. Similarly you may wish to devise a number of discussion activities for your course, but only use a part of them in any semester.