Some important concepts:
- a site collection is a unit of information belonging together; if information should be isolated/secured, it could be a separate site collection;
- having multiple site collections might require some additional development (e.g. packaging content types or site columns that you want to reuse in different site collections in features);
- a lot of built-in web parts only work inside a site collection, not across site collections;
- it’s good to have separate site collections for a corporate, governed intranet and the wild wild west of collaboration 🙂