Startups touting the next big thing can rake in millions while offering not-so-imaginative ideas such as a grilled cheese restaurant chain or a "Dropbox for physical storage" (Which is also, you know, just a regular box.)

But tech at its most enterprising can build whole new economies around brand new industries. Google did it with search; Amazon with online retail; and Facebook and Twitter with social networking. These days, however, the buzziest startups with the highest valuations—think Uber and Airbnb—aren't just the ones challenging incumbent businesses; it's almost an assumed part of the deal that at some point the most successful startups will have to go up against the government.