The Evolutionary Path
Viruses have obviously come a long way since then. One interesting thing to note is the change in medium. Those original harmless viruses, and the malicious ones that followed, relied heavily on infecting a medium. Viruses moved from computer to computer by adding themselves to floppy disks and hitching a ride to a new station. This would stay a popular method of transfer up until the early 2000s, when exploits over the Internet came into popular.
It didn't take long for things to get slightly more advanced. Viruses soon began to infect word documents and excel spreadsheets through exploits, which made it far easier for viruses to conceal themselves. It also didn't take long for email to get in on the action. Microsoft Outlook was a popular target for new malware.
A popular choice was to send an infected attachment in the body of an email. Once it was opened, it would infect the computer and look at the current contact list in Outlook. Common practice was to pick off the email addresses of the top 50 contacts and then use the infected account to send out more of the infected emails. Since it would look like a message from a friend, the chance of opening the attachment was higher. This process could then continue and infect large numbers of machines. Major viruses like the Melissa virus  and the Love Letter virus , used this format.
The closest thing to a true advancement would be a little variety. Malware programs began to take advantage of new technologies, like cross-site scripting, to run their programs on otherwise safe websites. The ultimate goal was for a virus is to be able to infect a user by simply getting them to load a script. Infected advertisements and hacked websites became a real factor for security conscious people.
The old tricks seemed to stay fairly popular though. Each of these are still threats in their own way, usually toward achieving a few objectives. Some are merely pet projects, existing for the sake of existing. The latest batch has mainly been made to seek a profit though.
Common trends are quite varied. Some stick to simple identity theft, often by recording passwords for later use or sale by the criminal. Others seek direct extortion, by using pop-ups and other tricks to convince the user to pay for a fake virus cleaner or registry repair tool. Search redirect viruses will route your Google services through other search engines to lead you to infected sites. The last section seeks to capture the actual power of the system itself, by adding computers to large botnets which can be used to send out spam or carry out other illegal actions.