I've learned a few things about "sandbox" campaigns, at least from my side of the table.
First, they work better when players have a few levels (and history of the campaign) behind them. That isn't much of a surprise, as sandboxes are largely player / character driven, and there has to be some history and investment before the desire to spread one's wings with an actual direction takes place. The Saturday night group is about to hit this.
It also requires frequent play and a steady roster. My Friday night group is, by it's intentional design, the exact opposite of this. Once a month more or less, with a changing roster (but pretty much a set core), it is not made for a sandbox. Maybe layer it will evolve into one, but with the slower leveling and the "drop in / drop out" nature of the campaign, I suspect not.
I'm beginning to see how something like Barrowmaze is an ideal adventure location for the "drop in / drop out" style of gaming.
Idle post breakfast thoughts on a rainy Thursday morning...