A couple years ago I went to Detrioit with Terry for the first high school reunion she'd ever been to (or heard about). She had a ball. A few weeks ago she came with me to my reunion--actually a multi-class picnic--which was the first one I'd ever heard about. That was also great fun. Neither of us felt at all melancholy about hooking up with people we'd known, finding out what they'd been up to, etc. It's more like an adventure, and a confirmation of life's possibilities.
I've been to a few college reuinions, and while it's true that they can be more impersonal because of larger class sizes, I think the trick is to contact old friends ahead of time and get a group together, so you know you'll have a core "gang" to hang with. Then when you run into others, it's a bonus. I'm lucky that a whole bunch of our college gang has stayed close over the years and we see each other fairly often with or without reunions.