Now that iOS 7 is out, you’ll find a whole load of applications have also been updated, and require downloading. Sometimes you’ll see the download simply stop, for no apparent reason. The reason is usually some app getting stuck: either part way through, or even right from the git-go.
Click on the downloads arrow (next to the search field in the upper right) and the downloads window will open. Find the one that is locked up, and hit the pause button next to it. Let the rest of the downloads proceed, and when they are done, go back to the troubled on, and resume the download.
This works for me somewhere around 99% of the time.