Prior to DropBox, no one did care about file synchronization, because the solutions presented thus far were largely "architecture astronaut" solutions. Rather than providing a simple, straightforward way to synchronize files (such as what DropBox provided), technologies like Hailstorm always focused on way too broad a feature set - it wasn't about "your files are always available", it was about pervasive cloud architectures that revolutionized the way you use your computer.
It wasn't until DropBox simplified the problem to "put files in this folder, and then they're synched" that file synchronization tools took off in any significant way.
Joel isn't complaining about file synchronization per se, he's arguing that the Microsoft approach of attempting to revolutionize computing every year with yet another failed omnipresent platform.