Bevan, the main difference is the workflow, and the methods for organizing and finding your images. If you ever only work with a single folder of images at a time, then as stated above Bridge will likely fit the bill. But if you ever need to work with sets of images that span multiple folders, LR is the tool of choice. For instance, I recently had a project which involved 12 images from Northern California that I wanted to put together in a calendar. All my images from North Cal span multiple years and are located in different folders, on different drives. No problem. First, in LR I typed "Northern California" in the Find box to quickly bring up all my North Cal images. (All my images have location keywords.) Next, I picked the 12 images I wanted and created a new collection in LR called "Calendar 2007" containing these 12 images. Then whenever I need to revisit this project, I just click on "Calendar 2007" in LR's list of collections and the 12 images come up right away ... even though they're all stored in different places.
Keywording and printing are also more convenient in LR, I feel.