For me it is the integration with the Finder. I have dropbox folder on my desktop, in there I have sub folders for different clients, proofs, etc. I can drag and drop as I would for any other folder on my desktop, it looks identical save for little green check marks beside it to show it has synced. Super simple to add a new folder as again it is just like the Finder, right click or keyboard shortcut and make a new folder, that is now in Dropbox as well. I love the file request feature in Dropbox, super handy and makes it super simple for the not tech people who need to send a large file but get confused with the online transfer sites. I create the file request link in Dropbox online, it gives me a link, I paste that link and email it to the client. When the client clicks the link a window opens up asking them to navigate to the file they want to send and that is it. Super simple and always works. Had mixed results that were not as clean and simple with others.
I use Dropbox to also backup my moms photos on her iPhone. So dropbox is setup on her phone, when she takes a picture as normal it saves to her photos but also saves to the dropbox folder I created for her so everything is auto backed up.