If I read a Hub that fits any of the following criteria, I don't hesitate for a second to flag it for admin.
Adult - Contains nudity, lewd or provocative images, excessive profanity, sexually explicit content, or contains links to adult or pornographic sites
Spam - Designed primarily to promote other sites, especially hubs containing unrelated links, links on duplicated content, or more than two links to a single site
Abuse - Has been promoted using spam, including email, comment, blog, wiki or other types of bulk unsolicted messages
Low Quality - Is very short, contains a large number of broken links or videos, or consists of unoriginal, nonsensical, difficult to read, or purely personal content
Miscategorized - Incorrectly categorized, has a misleading title, or has unrelated or excessive tags
Not written in English - Primarily written in a language other than English
Gambling - Promotes gambling or links to sites that promote or offer gambling for money
Sadly a lot of people in this thread have said they just vote Hubs down for several of these infractions, which only winds up hurting HubPages in the long run.