First, let me wear my moderator hat and make a few general statements:
- When you discuss a post on meta, leave a comment on that post to notify the author and other interested parties.
- Editing out 80% of an answer is generally considered vandalism. There are edge cases where it can be appropriate (for example, editing out plagiarized content, or editing out irrelevant material from a community wiki post), but most of the time, it would be abuse. Authors are entitled to their style, even if that includes droning on.
- If you flag a long answer, don't flag it as “not an answer”, as our (moderators') reaction is likely to be that you must have flagged the wrong post. Use a custom flag to explain how this answer does not address the question at all. Remember that the moderator may not be familiar with the material being discussed.
Moderator hat off now, speaking as a member of the community (both of this site, and of Stack Exchange in general).
What you consider irrelevant material may be what other people consider interesting side remarks. An encyclopedic (but relevant) answer is what we strive for; a two-line answer is what we make do with sometimes.
If you think an answer is completely irrelevant, flag it for deletion (with a custom message, see above).
If you think that an answer contains some relevant material but that it's buried under other considerations, feel free to edit that answer to make it easier to locate the directly-relevant material. Do not edit the side notes out! Use linking words, paragraph breaks, titles, emphasis and other language and presentation tools intelligently.
If you don't feel up to the task of editing, leave a comment stating that you had a hard time reading the answer in the context of the question, and explain the kind of editing you'd like the author to do.
If you think that an answer is correct but very badly written, it's a valid reason for downvoting. Please leave a comment explaining why you downvote, and ideally come back after a couple of days and change your vote if the author has improved the answer to your satisfaction.