They will probably have a strict policy of banning people for posting keys or just plain remove the ability to leave descriptions. I could actually see TOR being used to share the keys and people downloading the encrypted file and going through the one extra step of copying and pasting the key into WinRAR/7Z/... Of course if a user can do it, then a program will be made that does it automatically. I imagine you'll just register a protocol in your browser when you install the client called 'mega:' much like mailto/ftp/http/https/irc/... In fact, there already exists the ability to have login details in a URL so I don't see why this can't exist for mega: links. Then the URL handler will pass off the address (with the key) to a downloader that strips the key, requests the download, and then decrypts the saved file, all without user interaction.
Any idiot knows that lobbying is a bribe by another name. Judges and police and prosecutors: 'Civil Asset Forfeiture' laws are just one way to pay off judges. 'Eminent Domain' has been abused to death against people who just wouldn't make the city enough tax money.
Swamping Mega with takedown requests if for the explicit purpose of denial of service, could actually be treated as harassment. Legal spam can get you disbarred. Dodgy bill collectors with 'flawed' records and threats have actually been prosecuted in many states in the US.
One workaround to the issue with those huge files with Freenet is to have chunks 32KB or so in size. Your client reads a header from the first chunk and then if the file is too big to fit in that chunk, it'll find a recursive list of chunks until it downloads the entire file. Essentially the chunk can contain a manifest. The nice thing about this is that you don't have "millions of multigigabyte media files" that are likely media files. You have trillions of chunks with only a limited set of possible lengths. Oh, did I mention that each chunk has decryption keys in the previous chunk in the chain? And that the first chunk is also encrypted. If you don't have the original keys, you have no idea which files your node server is caching. AKA the RIPA loophole - otherwise ISP operators could be arrested for refusing to provide keys to your HTTPS sessions.
A neat trick is that once someone posts an encrypted file to these servers, it's in theory trivial to have local mirrors like Bit Torrent/Freenet/Waste. This would be the foundation of a true Internet 3.0. Newer routers could be designed to run some applications on this backend, transparently. Right now to use something like Freenet, you have to be kind of technical. If you could just buy some box that provided similar capabilities, it would catch on like wildfire. Of course, Freenet is probably a bad example since it's considered the slowest (and likely most secure) of darknets. I2P or chained OpenVPN with a distributed filesystem is a better analogy for the backend.