As a developer of a sound-based app (100sounds.com), the most common complaint is "I'm not hearing any sounds!"
Without fail, they have their iPhone Ringer Switch on MUTE.
When they email us, our auto-responder tells them to check that, and we also follow up. Many of them don't even know they have such a switch. Also, to be fair, not all apps obey the switch, but our app does, so they might be conditioned into thinking it only applies to ringtones.
But the problem is the folks who DON'T contact us and post a bad review. Even if they figure it out later, they don't update the review.
So, bottom line, if you have trouble with an app, visit the developer's website, where they have a lot more room to offer help (iTunes limits you to 4,000 characters and 5 pictures). Our site has a Flash demo, FAQs, etc.