There were a few sites like this and none were really used because they weren't integrated in the game. Uploading a save or browsing and downloading them needs to be possible from an easy to use gui in game.
This would be cool. The ability to upload saves from ingame would be super, especially if it was able to simultaneously post a list of installed add-ons. After all, sharing saves is pointless if the recipient doesn't have the right bricks/colorset/events.
If things were to be uploaded from ingame it would definitely need some sort of moderation.
It seems maybe easier to only have uploading from outside the game, but I wonder how many would bother. It's quicker to just push an upload button while playing.
Maybe a better system is an in-game cloud storage service for saves with a website you can access for sharing the saves.
All of your saves get backed up to a cloud server. You can redownload them as necessary.
However, if you go to the website you can make your saves public, or just some of them.
Others can then download them.
There could even be community highlights or editors picks.
I'm just openly brainstorming here.