I'm still not sure how much of an advantage that is to the average user. To people on HN it's a huge plus, but to real world users...
Regardless DB should be able to carve out space where cross platform is important, but I can see their market size shrinking when Apple and Microsoft really start pushing their cloud storage solutions. Unless of course they get to a Facebook like level of critical mass.
Even you aren't cross platform personally (e.g. you don't have a Mac and a PC at work or an Android phone or ...) Dropbox lets you share files and folders with other Dropbox users. Unless you're confident that you'll never want to share with someone on a different platform (which only sounds realistic if you never want to share at all), that's important even if your personal environment is homogenous.
I'd guess that most Dropbox users share something (though I haven't seen statistics on that) so they are probably less threatened by vendor-specific solutions than you might think. I suspect Dropbox is more threatened by services like Amazon Cloud Drive (especially since Amazon is an infrastructure provider for them) than they are by anything Apple or Microsoft has planned.
"To people on HN it's a huge plus, but to real world users..."
I think the opposite. If HN readers tastes' are similar to the average founder in the Valley, they are vastly more likely to use Macs and iPhones together than the average real world user, who is much more likely to use Windows/iOS or Windows/Android.
But the cross platform advantage means it get's recommended by the type of people that have cross platform needs to the type of people who don't. Would you rather hear, 'if you have an iPhone and a mac' or 'it just works.'
When I was in graduate school, I saved all my documents to my dropbox folder, which was sync'd to my school laptop (OSX), my personal laptop (Windows), a lab computer where the stats software lived (Windows), my advisers computer (Windows), and my ipod touch (iOS).
I wasn't anywhere near the only one who had those kind of cross-platform needs. My program was a real mix of Windows and OSX users with a few linux users for flavor.
Not sure about that. I think in that case it just needs to work, without any configuration, installing etc... It needs to be as pain free as possible. Unless DB can somehow be installed by default on those devices I can't see it making the grade.
Dropbox is pre-installed on many Android phones, for example:
http://geektech.in/archives/6318 (which comes with extra free storage up to 5GB for HTC users).
Pre-installing on PCs is possible, though my quick Googling didn't turn up anyone doing it. Pre-installation on Apple devices is unlikely, of course.
Remember when the iPod only supported Mac? What happens when Apple decides to change the photostream shared folder on the PC into a generic folder to sync all files into the cloud? If Apple decides to roll that out, Dropbox days will be numbered. BTW, I'm a big fan of Dropbox, I just think the future is pretty clear in terms of how easily they can be marginalized.
Regardless DB should be able to carve out space where cross platform is important, but I can see their market size shrinking when Apple and Microsoft really start pushing their cloud storage solutions. Unless of course they get to a Facebook like level of critical mass.