OK, let's dispense with the elephant in the room.
Yes, we know that Facebook is the de-facto platform for classmates to stay in touch. But not everyone belongs to Facebook, nor do they want to! Privacy concerns and being inundated with minutiae are just a few reasons. Time magazine had a recent article about Facebook backlash.
In short, we see 3 components to a reunion website: tools for the planning team, pre-reunion information sharing, and post-reunion wrap-ups.
Planning team: Streamlining the thankless work of these volunteers is vital! We often see the team divvy up the class list, each responsible for contacting a subset of the class. Excel spreadsheets are usually the tool of choice. One unfortunate soul gets to manage the master spreadsheet, trying to compile multiple files into one. Copying and pasting, interpreting each other's notes, responding to some goofy color-coded scheme. Praying that they don't overlook someone. Sigh...
How about a secure, on-line master database? All the information in one place? Editing rights determined by agreed-upon login credentials? Yeah!
Pre-reunion: Post information about upcoming events to the site, including directions and maps. Allow classmates to offer activity suggestions. Provide the ability to donate or pay on-line.
Post-reunion: Create guestbooks and message boards to allow classmates to comment and share memories. Post photos. Keep the community alive to make the next reunion easier to plan!