Hosey posted a good checklist on things to think about when doing Mac OS X applications.
In the comments, I added a few tips for talking to and dealing with the customer/user that have worked well for me, and I’ll reproduce them here:
Have big ears: Listen to all feedback. Realize that people aren’t trying to beat your app up when they’re being negative, and that an app that’s just right for a selection of people will make those people happier than an app that takes a swiss-army-knife approach solely in order to please the critics.
Say no: Make your users work hard to win you over. Don’t implement every feature that’s requested of you unless it fills a gap in your application without changing its intended goal. Think long and hard about adding features that’ll extend your app’s intended goal or even add a second goal. Consider offering a level of indirection like scriptability if people want to do lots of different but fundamentally similar things and it could fit in with the rest of the app.
Be swift: Do not pile up customer requests. Answer them right now. Even if you don’t know the answer, let them know you’re listening and that you’ll be looking into it. Allow for a maximum of 5 to 15 minutes of research, depending on how much mail you get. If the fix is a quick fix on the current release, send a special build and ask “does this fix the problem? could this be done in a better way? is this what you had in mind?”.
Follow up: This goes hand in hand with the previous tip. Follow up, follow up, follow up. Did that work for them? Have they seen the bug since the last email? Was it properly fixed in the build that claimed to fix it?