Year 2004 was split from the middle: the beginning of the year was record-breaking busy with a huge number of major new virus outbreaks and the "virus war" between Mydoom, Bagle and Netsky. After June, things calmed down, and we've only seen some outbreaks since.
Other developments: open-source botnets, massive phishing cases, big increase in professional virus writing. Number of known viruses passed the 100,000 mark. First real mobile phone viruses were found. Spamming is getting worse and worse - and more profitable for spammers.
On the other hand, year 2004 was the best year ever in actually catching virus writers and other cyber criminals.