What is an Opportunity?
I have found that the concept of an Opportunity and the Opportunity Record can sometimes be hard to grasp. What my standard line tends to be is that an Opportunity is actually the tracking of the dollar value or the actual "sale". This sale has a potential of close and for any given Account or Contact you can have unlimited opportunities.
One thing to keep in mind with regards to opportunities is how they impact the sales analysis reporting. Consider this when creating an opportunity. Are you comfortable enough with the realistic expectation of close around that opportunity to report it all the way up to perhaps a venture capital team. A team that (even if you assign a 5% probability) would expect those dollars to be considered revenue even at 5%.
If you are dealing with a sales funnel with thousands of prospects I would argue that not every prospect should be classified as an opportunity. At some point they cross the line, but until that time you don't necessarily want to have a huge dollar value of potential mucking up your weekly reports.
In general people tend to fall on one side or the other in this argument. It is really a matter of style.