I would like to highlite the points published by author Stowe Boyd (June 01, 2006), and encourage you to visit and read the entire article (click here). The article is based on an interview with Carl Tyler (CTO of Instant Technologies).
While these may seem obvious, they do represent a major source of confusion and frustration among IM users. If you are interested in a more formal (and somewhat edutaining) guide to Instant Messaging, check out The Instant Messenger Handbook (published at Black Table).
David