I basically want to share a folder in Dropbox but I don't want that others have read-write access. How to achieve that?
Unforuntatelly, your shared folders are r/w. However, you can workaround that using cloudHQ.
Basically, lets suppose you want to share folder FOLDER_TO_SHARE. You need to create an extra folder (lets call it FOLDER_TO_SHARE_READ_ONLY) in Dropbox and then establishing cloudHQ one-way sync (and with option "override changes on target") from from FOLDER_TO_SHARE to FOLDER_TO_SHARE_READ_ONLY and then you can share FOLDER_TO_SHARE_READ_ONLY.
So cloudHQ will do one-way sync from FOLDER_TO_SHARE --> FOLDER_TO_SHARE_READ_ONLY so only changes from FOLDER_TO_SHARE will be propagated to FOLDER_TO_SHARE_READ_ONLY. And since you have "override changes on target" set, then any outside change on the FOLDER_TO_SHARE_READ_ONLY will be automatically reverted back.
Some details can be found here: http://blog.cloudhq.net/post/40164161550/how-to-create-read-only-dropbox-shared-folders-using
Your public folders are read only, your shared folders are r/w and there is noway to change that (yet). See dropbox help