A fast lens enables you to take a better picture in a lower light environment than a slower lens. Digital zoom enables zooming through the use of software. Optical zoom provides a far better image than digital zoom. It does not, however, accurately represent the image that you're taking. In contrast, a LCD is a television-like screen that shows you what your picture will look like.
Serial cables are the slowest by a relatively high factor.
Different formats also have implications for quality and the capability of the file to be used and viewed by software applications. If you need more powerful flash capabilities, find out whether the camera can be used with an external flash. Often, digital cameras that have this feature also offer slide show capabilities. Generally, you find out about the surprise features only after you've purchased a camera, shot a hundred or so pictures, and have finally decided to sit down to read your user's manual. Such features typically are neither written up in the comparison tables nor included in the feature lists on the retail packaging, and you probably won't find a salesperson who'll be able to tell you about them.