Well, the original comment made 3 distinct arguments which can basically be summed up as:
1) Its feature-set is easily duplicated (from a technical perspective)
2) It doesn't solve one particular use-case (that it wasn't designed to solve)
3) It's hard to imagine it catching on, let alone making money
Point #1 was widely mocked after Dropbox caught on, but turned out to be kind of correct - it just wasn't end-users that mimicked its feature-set, it was other companies.
Point #2 is irrelevant.
Point #3 was hilariously wrong. It went viral as hell, and the founders could've rode off into the sunset with a boatload of cash if they sold the company to Apple when they were approached. In the alternate universe where that happened, Apple devices just have a Dropbox feature instead of iCloud, a la Siri and Shazam.