The Sandbox really does not belong on Main and should stay on Meta
Let me review the arguments made for moving the Sandbox to main (from the question):
- Users with less than 5 reputation can still use the sandbox. (This is the most convincing argument, in my opinion.)
The reputation requirement has been removed. This point is no longer valid.
- The visibility of the sandbox is increased. The majority of users do not regularly visit meta, and a question that is probably going to be at the top of the "active questions" list almost perpetually is much more visible than a single line in a small sidebar box.
Main is much more active than Meta. At this time of writing, the last modification to any post in the Sandbox was 2-3 hours ago, and it is the fourth question on the front page of Meta. A question from the same time frame would be #16 on the front page of Main. The Sandbox, if it were on Main, would definitely be bumped up to the top often, but it would be pushed down quite rapidly too.
Besides, this is no guarantee that new users will actually use the Sandbox. The majority of new users don't actually look around and get a feel for how the site works. I would think that, if anything, a new user would be much more likely to be confused because the Sandbox is so different from everything else. The vast majority (I think) of users who post well-received challenges use the Sandbox, and any user that posts a poor or could-be-better challenge typically gets a link to the Sandbox, so I think we just need to keep doing that. That's how people find out about the Sandbox and the (positive!) peer pressure makes them use it.
- It would make more sense there anyway; meta is for discussion about this Stack Exchange site, while main is where the programming puzzling and code golfing goes.
Actually, I would consider this to be more an argument for keeping it on Meta. There is lots of discussion and no programming, puzzling, coding, or golfing going on in the Sandbox (or at least, very little). See the paragraph below for more.
Arguments for keeping the Sandbox on Meta
- Meta is the right place for the Sandbox.
On my home site, Christianity.SE, a perfectly valid use of Meta is to ask something like "Would it be acceptable to ask this question: X?" (One, two, three examples.) In my opinion, the Sandbox fulfills precisely this purpose. It's where users can ask "Is this a good question for the site? Is it an appropriate question for the site?" and get feedback on their ideas before posting them on Main.
- The Sandbox is featured in the sidebar
I don't think anyone has brought this up, but consider this: because the Sandbox is a featured post on Meta, it is (relatively) prominently displayed in the sidebar near the top. If we moved the Sandbox to Main, we would lose this.
So what should we do?
I think the biggest problem with the Sandbox is that Sandbox posts get little attention. This is not a technical problem; it is a behavioral problem. I seriously doubt that moving the Sandbox to Main will fix this. It might even get worse because then there would be more posts and probably not as many people reviewing them (proportionally, not necessarily absolutely).
I have a suggestion: lets encourage one another to review Sandbox posts. Every day, look through the ten most recent (by creation or modification) and vote up, vote down, or comment. We have a great spread of time zones that our users live in, so I think this would work pretty well.
I volunteer to lead this effort if it means the Sandbox stays on Meta.