I have a friend on Facebook, who happens to be a publisher, and he posted a comment yesterday about how detrimental it is for authors to post disparaging comments about, well, anyone, but he focused on publishers. I don't know if someone said something bad about his company or just in general, but he did have a good point. The publishing world is a small world. You might not believe it, but editors and agents do talk to one another, and if you bad-mouth them long and loud enough, you will blacklist yourself.
I know the writing/publishing world can be difficult. I know you get rejection after rejection after rejection and it eventually wears on your good nature. I also know you sometimes get an agent or publisher who thinks they are being helpful and rips your story to shreds. I know how hard it is to resist the temptation to tell the world what an idiot those people are.
The internet has made it extremely easy for everyone to tell the world about their grievances, but it's not always a good idea. Say you post negative things about every person who ever rejected a story, then one agent/publisher decides they like your work. I guarantee you, one of the first things they are going to do is check out your web presence. If they see that you've bad-mouthed people before, they might not want to take you on. It's a matter of principle and professional courtesy.
Now, that's not to say I haven't had my share of bad things to say about agents or publishers. I have. But I try not to name names. We're human, so we can't expect to be happy and nice all the time. What you need to remember is that the publisher or agent is not attacking you or your writing personally, they are simply making a business decision. Talking bad about them or cussing at them is not going to change their minds, it's just going to piss them off. If you really want to get even, keep writing and get better, try for that next bestseller. Then, when it sells really big, you can rub it in their face (I don't really recommend doing this, either. No one likes a gloater, but it sounds good in theory!).