It would be incredibly complex to build your own Dropbox. Maybe it wouldn't be "complex" to use rsync to automatically copy files over to some other drive if that's all you wanted, but Dropbox does way more than that and I don't plan on dropping Dropbox for some half baked solution that requires more effort to set up, maintenance and ends up in lost data because on the one in a million chance the FBI might want to look at pictures of my pets.
I don't know how I feel about government surveillance, but I sure as hell don't see it worth to trade some supposed imperceptible theoretical harm with tons of actual effort and inconvenience.
You don't have to write your own Dropbox. You just have to host one.
And even beside the question of government surveillance there is the advantage that it is under your control, not somebody else's. File sharing is so generic that the lockin opportunity is less than it is in other domains (like social networking) but there still can be advantage to being the owner and not merely a renter.
And beyond just being a subset of the features, it doesn't have Dropbox's ecosystem. Can you auto-sync your Oreilly Media books automatically with own cloud right from the Oreilly website? Does 1Password automatically sync with it? Is there a screen sharing app that automatically pushes to owncloud? Does it have a push API? IFTTT support? I, and others, use all these with Dropbox and it probably doesn't make sense to give them up because the NSA is the boogeyman FUD.
As you mention, hosting OwnCloud removes your cat/fire scenario. Even by hosting it at a US ISP, you are significantly reducing the likelihood that the government can index or be two clicks away from your data, and your host (unlike Google/Dropbox) isn't likely to be mining your data for future business models. Hosting it at an ISP in a country that respects individuals privacy (e.g. Iceland) means your data won't be in the government's hands unless you're directly targeted by a state actor.
A subset of the features is, for me, an advantage. Hosting my password database myself is a huge feature. My Mac's built in screen capture writes stuff to disk. I can set that to be the Owncloud folder. No third party app required! :)
Diaspora*
> and a search engine
DuckDuckGo
I suppose that any single pod could be compromised fairly easily, and some of the larger pods have a large number/portion of users, so just compromising some of the larger pods could be sufficient.
Also, iirc, Diaspora* has been said to have some security and privacy concerns,
But I thought it would be good to mention that there isn't really a single "Disapora*" which can be told to give up all the data for all users, because different users use different pods. (And I think a collection of pods can be somewhat isolated from the rest maybe? I'm not sure.)