The most important thing I have found as a web user and working in new media is creative. It used to be easy to make a banner ad and get someone to click it, but as targeting becomes easier and easier, the creative can become more challenging.
Parts of it are easier, as the more you know about your target, the more you can tailor the message, but integrating the messaging can be more challenging.
Another challenge is of course, the fact that the medium is constantly changing, illustrated by YouTube's new video advertising, and Google's new video advertising on their results pages.
