The Sound of Silence

You need great sound on your film.  Not good sound.  Great.  It doesn't matter if your flick is one of the best looking things to hit the screen in a while.  Production value won't matter.  If your sound is crappy it's going to distract your audience and they will not care for your movie.  Bad sound is annoying.  It grates on people.  All people.
As a rule you should never use your on camera mic for your primary audio.  Cameras never have great mics and they usually take in every sound in the world making for very cluttered audio.  Plus all cameras make internal noise.  Even digital ones make sound when recording.  When filming my DSLR cameras make a clicking sound as the internal mechanisms do their thing.  Those clicks come through onto the audio because all that internal noise is coming from a place right next to the very sensitive mic in the camera.  I've heard this noise on other short films and that's a dead give away that they used the on board mic.
Another issue with the on board mic is that the closest thing to the mic is going to be the loudest thing on the audio track.  That's not going to be your actors lines.  It's going to be your camera man breathing and grunting as he gets the shot. 
On my films I do a quick pass on the audio and then hand it off to someone who knows what they are doing.  I shoot my audio separate from my footage.  It's either going into a sound mans recorder or into a digital recorder.  If I do record sound into my camera its with a different mic than the one built into the camera.  I'll take that audio from the separate recording device and sync it up with the crappy on board sound and make sure that the audio is all at the same level.  That's the extent of what I'm gonna do.  I'll send the project to someone who does sound engineering and what I get back is a clean audio track that highlights my actors voices.
I've seen many an indie flick that hasn't taken the time to work their sound properly.  Ambient noise changes dramatically from one shot to the next giving the audience an annoying dose of random humming off and on during a conversation.  One actor is way louder than the other even though they are standing right next to each other.  It's all a mess and usually that's the thing I come away with at the end of the viewing.
The important part to take away from this post is that all parts of your film are important and need to be treated with as much care and attention as the the rest.