In the Online world writing ads is only half the battle. Designing a good banner ad can be a time consuming job.
Considerations of what the message is, the subject matter, understanding of the target audience, what needs to be said in the ad, color schemes, fonts, graphics, and how it should be laid out and displayed are all important considerations for an effective advertisement. There has to be a good balance between getting the attention of a person, rather than irritating a website visitor with an excessive amount of motion in the ads themselves.
Some websites go overboard on the amount of Flash in Online Ads. The amount of repetition movements, or repeated text displays in a GIF animation file, for an effective online advertisement can be limited to three to five reps, depending on the situation.
Sometimes, the advertisement develops into an ongoing process, whereby the ad itself has to go through many rewrites before it is acceptable. I am reminded of the Go Daddy Girl story about how Bob Parsons and the Go Daddy company had to go through many rewrites of their television commercial for the Super Bowl. They ended up going through a dozen or more takes, with different ideas, storylines, and actual advertisements before the Network would allow their commercial to run at the Super Bowl.