There are many user behaviors that have been ingrained in us over time. Some of them good, and some of them just because, "that's the way that it has been done for years, people expect it." The problem is, in some cases you can come up with an easier or more sensible way to do something. An example:
Apple has changed the orientation of scroll bars to match that of their touch interfaces for OSX Lion. They have changed the scroll behavior from scroll down to move down the page to "pushing" the content up to scroll down the page, which is the opposite to what most of us have been doing for our entire lives of computer use. Based on some user feedback, some people resist it and some love it. Although, most that initially resist it have come to prefer it in the end. For those who absolutely cannot get past the change, Apple has allowed users to set it back to the old way if they must.
So the question is, when is it ok to challenge an accepted convention when you have something that you feel is better? Conventions should not be challenged often and in a way that is just change for the sake of change, but when is it okay to implement something new that you strongly believe will benefit users in the end? Also how would you suggest going about doing this, as most changes would be met a fair amount of initial resistance?