Here are software tech trends that I thought were going to be HUGE but kind of fizzled. Some I strongly believe are ripe for a comeback:
1. (As before) I thought Java and Java Applets would be the GUI and OS for everything.
2. Object-oriented Programming. It has become fashionable to ridicule OOP but I still think OOP (minus nearly useless inheritance) plus Design Patterns is powerful, just not the grand solution to all problems.
3. Knowledge Management Systems. There were supposed to be rules engines for every domain of human knowledge that experts would fill with rules-of-thumb and facts and voila! You'd have all-knowing oracles that could diagnose any medical problem, etc. They still exist but proved expensive to build and limited. IBM's Watson revived the idea for a while, though.
4. Associative Operating Systems and Object Stores. File and folder OS'es were supposed to be dead and gone by now, replaced with content-oriented OS'es that retrieved information by their contents and by tags without regard for location. Everything was supposed to be a series of linked and nested objects so that if you updated a spreadsheet here, your related contract and presentation would update automagically.
5. Syndicated content and automatic agents. There was a time when RSS was king and existed as a parallel API, if you will, to human-readable content. Everything from your coffee pot to your favorite news site would have a feed and your self-programmed agents would hunt and sift and deliver information to your dashboard, handle routine transactions for you and, of course, broadcast your status and thoughts to the world. Much of this, too, exists in scattered patches but isn't the universal nerd nirvana we expected. Instead, people relinquished rich, decentralized RSS feeds for controlled, predigested Facebook, Twitter and Reddit feeds. Sad. APIs and automation are big as ever but somehow never became tools of the people.
On the other hand, I knew CGI would revolutionize movies, 10 years before Pixar. I showed my traditional animator friend early graphics and he laughed at the idea. I adopted digital photography years before film cameras were dead. I also bet my career on graphic user interfaces when text-mode MS-DOS was the only serious operating system and later turned down a higher paying client-server job to work on this crazy WWW thing.