Well, the iPad has many more apps (at least for the moment) and a bigger screen, both of which make it more suitable for light-to-medium work stuff.

For example, I can perfectly imagine (and there are many cases of this) a college student passing up on buying a laptop for class and opt for an iPad with a bluetooth keyboard instead. In this scenario, this setup can effectively replace a computer for most students most of the time.

However, I don't see how a kindle fire can fit into that context.