I like the idea, it's like uservoice, getsatisfaction etc only really really simple. My only complaint is the fact you've called it 'dropbox' most people associate that word with a file storage/sharing device (such as getdropbox.com or the OSX dropbox). Perhaps 'comment box' would be more appropriate? Also, you may want to add some flood control - I was able to bombard my test account with all manner of crap ;)
I had to chuckle when the 'create a premium dropbox now!' button took me to... a dropbox! You're taking the MVP approach pretty seriously, aren't you?
I can also see the benefit of providing some embed code to allow users to integrate it on their own site.
Good job on the design, BTW.