What you can delete
I was googling what files I could delete, couldn't find any satisfactory answers, so I took the risk for some folders through trial and error.
This is what I know about some of the files:
SAFE TO DELETE
DCIM (in here are pictures you took with the phone's camera)
Download (you will lose the things you have previously downloaded)
media (you will lose files you have in here)
Movies (you will lose files)
Pictures (you will lose files)
Playlists (you will lose the playlists you have in your music player)
Music (you will lose files)
FOLDERS CREATED BY APPS: So when you delete these files, it may affect the performance of the respective applications.
I'm no expert, but hope this helps