What if Dropbox's datacenter burns down?
Since Dropbox files are stored locally on your sync'd computers as well as in the cloud, your house and Dropbox's data centers would both have to burn down to lose all your data. This also saves your data if they suddenly go out of business.
Another plus of Dropbox that I don't see mentioned so far is public access folders. I can drop a file into the Public folder and right click the file to get a publicly accessible link.
Also, if you upload a file anyone else has uploaded to the cloud already, it figures out (via black magic, I assume) it already has that file and doesn't upload it. Instant sync, doesn't waste upload bandwidth, and allows your other sync'd computers to instantly download it.