Dropbox is for personal storage AFAIK. I think the idea behind "Mega" is to allow filesharing with
an added level of security. i.e. The user can control who has access to download the file, but
only the user (original uploader) is able to delete or modify it on the website.
Ok... So it stops govt etc from seeing what files are on there but it doesn't physically stop them shutting it down. It makes the legal side a bit tougher because previously they would have had to get a warrant and then once the servers are seized they'd have all the proof they needed so now that's gone it just makes their job tougher but not at all impossible.
There are already services that are 'like Dropbox but with security', e.g. SpiderOak ... (Dropbox's security is so bad I want to 'correct' your sentence to "So it's like Dropbox but with security")