They're doing the right thing by keeping it simple as long as they can but I'm fairly sure they're savvy enough to have brainstormed each and every bit that they could add / leave out and their decisions so far have been to keep it lean.
It's smart because that way the bandwidth bill on the free tier is still within bounds, as soon as they pull out all the stops they can go head to head with any content store on the net based on the files they already have in their inventory.
I'm in no way privy to anything going on at dropbox and I have no stake in the outcome but if I were running a service and a formidable competitor announced their entry into my little empire I'd be gearing up for war.
Similar to email vs. Facebook, while there are lots of files people want to share, the reality is that _most_ content individual to users (i.e. not music downloads or software packages) is fairly private stuff.
(I'm very restrictive when i do have to login. I do what i'm there for. Nothing else. Then quickly logout.)