YouTube videos tend to be brief (even with the newly-relaxed upload limit), but even so people often stop watching partway through if they get bored. Hot Spots, a new feature on YouTube, helps you identify when the audience is getting turned off by showing how likely people are to watch at any given time, based on viewing statistics from YouTube's Insight tracking feature. To access the Hot Spots graph, select the Hot Spots tab from the Insights dashboard. (Like many Google features, it seems to be rolling out gradually; I couldn't see it on my own YouTube account).
While any judgement on your work might need to be taken with a grain of salt if the video has only been viewed a handful of times, spotting a common exit point could suggest ways to re-edit your video for improved impact. If you want to share someone else's YouTube video and cut out the dull bits, check out recently-mentioned Splicd.