If everyone inconsiderately includes extended passages, it soon becomes impossible to determine who said what. This problem is exacerbated for subscribers on digests. When responding to a previous posting, include the name of the earlier sender so that readers are oriented to the issue. Quote only those brief portions of the previous message that are necessary to make your point. Never include the entire passage.
Most lists develop a particular style.
As well, lists often have favorite gurus who are well respected and liked. To obtain the flavour of a list, lurk for a few weeks before posting a message. You will discover whether the list encourages or discourages such practices as off-topic messages or jokes that are unrelated to the list's focus. Try to identify whether the list has a resident guru or two. Send messages that conform to a list's stated purpose.