There's been a lot of talk lately about The Rejection Queen.
This is a prime example of what NOT to do. Ever.
For one thing, it's rude and unprofessional to talk shit about agents and publishers. They're just people doing their jobs to the best of their abilities. A job that is completely subjective when it comes to taste. It is absolutely impossible to write something that everybody is going to love. So getting upset about a rejection is simply unproductive.
For another thing, the literary world is a very small pond and people talk. You talk smack about agent A and writer Q is going to tell them about it. And then agent A is going to tell every other agent he or she knows about what an unprofessional dumbass you are.
Even if you use a pseudonym, very little on the internet is completely private. With just a little bit of Google-power, several people were able to uncover The Rejection Queen's true identity. One wrong comment, one slip-up and the people you're bashing are going to find out who you are.
If that form rejection you just received on a full submission really bugs you to the point where you can't not say anything, then for Gods' sakes don't post your glorious nine-page screed against all things agently on the internet. Call a friend and rant about it to them, write it in a paper journal.