It's a tough biz, this indepdendent filmmaking. The film festival circuit may not be the hardest part, but it has its peaks and valleys just like anything else. You slave over a film, massage it to the best piece of work you can manage, and then send it off with a check to a film festival. And another. And another. And another. The thanks you get is usually along the lines of "thanks for submitting, but there were too many other films we liked better." For every festival that accepts your film there are a dozen who turn it down.
In the face of such rejection, it's natural to feel a little whiny. We can all be really good at complaining when we feel unappreciated, and misery loves company. Over coffee, at a party, or (most especially) on an internet message board in the company of comrades, the urge to grumble takes control and the snarky comments fly.
However: as legitimately indignant (and amusing) as the Bitter Man might be, here's why you might want to restrain yourself.
1. You don't know the whole story. Yes, the Bumbledyfloop Film Festival rejected your mind-blowing documentary short about that Olympic pole vaulter. And while it may be the very best doc short ever about pole vaulting, you submitted it the year after Bumbledyfloop did their big "Olympic Heroes" program. Their local audience has seen all the Olympic docs they can handle for a while, so Bumbledyfloop decided to pass. This is kind of a silly example, but the reasons for turning a film down are as countless as the stars -- and they don't always have something to do with the quality of your film.
I've heard filmmakers grouse about this, too: that festivals dare ever take anything into consideration but the quality of the films they program. My answer: welcome to the real world, kid. It ain't fair, but sometimes that unfairness works in your direction, so be grateful when it happens. Personally, I like seeing festivals that program imperfect films because they took a shine to them, or because they fit a theme. Oftentimes that's where the really interesting stuff happens in cinema.
2. You never know who's listening. People will form opinions of you based on what you say, whether you intended it to be for their ears or not. So whether you're at a festival party trashing that pretentious piece of junk you just saw or relating the story of a particularly disastrous screening experience you had at the Southwest Poughkeepsietown Cinema Celebration on a message board, consider how your words will sound to someone who doesn't know you. It's a small world; festival directors talk to one another, and they troll the same web sites you do. Your reputation will precede you, so don't hurt your film's chances by being known as a "difficult" filmmaker.
3. Nothing grows on scorched earth. Here's one I've seen a few times: a filmmaker receives a personal invitation to submit new work to a festival that has previously passed his movies over. Suspicious that the festival just wants his submission fee, the filmmaker refuses to submit -- or worse, actually voices his suspicions and/or hurt feelings. In the vast majority of cases, festival programmers are far too busy to solicit submissions from individual filmmakers unless they genuinely have an interest in that filmmaker's work. An invitation to submit is not a guaranteed acceptance, but it's a far better shot than not being invited. (I'm not counting the mass invitation e-mails that go out to previous submitters.) You've done something to merit the programmer's attention; don't squander it by holding a grudge.
(As a side note, while submission fees are a significant source of revenue for many film festivals, let me reassure you that the film festival that exists merely to collect such fees is an exceedingly rare animal, if it exists at all. Festivals cost far more to run than submission fees alone could ever support.)
4. You're better than that. Seriously. Unless you really are a creep (in which case I can't help you), you probably don't want to be perceived as one. This is particularly true when your career is on the line. Personality is a large part of the hiring process in any industry. If you want to build a career in filmmaking, you have to sell yourself along with your work. It's a lot easier to do that if you're known as the easygoing sort with a kind word for everyone. So the next time you're tempted to blow off some steam or give an incompetent festival worker a well-deserved thrashing in the filmmakers' forums, think hard about whether that's really the way you want to be remembered.
Do all the whining you want in the comments. I won't hold it against you. Promise.