The consensus also seemed to be that:
These deleted posts should be edited down to a simple link to the real challenge (presumably with the title of the real challenge as the link text, like this). This is so users with high-rep don't have the deleted challenges cluttering their screens.
Proposals that are duplicates or highly down voted or infeasible/impossible should also be deleted. Presumably their content can also be edited to reduce bulk.
Thus "abandoned" means something that has been forgotten or never completed, but still has the potential to make a good challenge.
Can everyone agree on these rules? What other details should be mentioned?
I want this post to serve as a definitive guide for what people should and shouldn't delete in the sandbox. An obvious problem is that you can only delete posts that aren't your own if they are down-voted enough (maybe that changes at some rep level, I'm not sure), so besides the editing this seems like a mods only task.
Side Question: As many of you know I make a lot of challenges but almost never post them in the Sandbox so I'm not the best judge of Sandbox mechanics, I just think it looks messy. Do you think Sandbox improvements like this will really help new (or old) users post good challenges?
Sandbox Deletion Rules
(I just made up the specifics, feel free to edit or suggest changes.)
#[Title of Deleted Challenge](url of posted challenge)
(What about comments?)
The following only applies to Sandbox proposals that have been inactive for more than 3 weeks:
Proposals that are clear duplicates of existing questions should edited with a link to the duplicate and deleted if possible:
[Duplicate](url of original duplicate)
Proposals with a score of -3 or less should be edited to reduce bulk and deleted if possible.
Proposals that were otherwise already deleted should be edited to reduce bulk.