Perhaps the largest example of this would be Wave. When I first heard about Google Wave, I was incredibly impressed and immediately signed up for the beta. I really did think it would replace email and IM--it was an elegant solution that did both, at the same time. I then played around with it, rather liked it and even got some friends to use it. I was still trying to use it well after almost everyone gave up on it.
I was rather disappointed in its ignominious decline and eventual death.
I also really liked webOS. All your apps would just be web apps! It would be great. It wasn't.
I really loved the Zune HD. It had one of the best interfaces I had ever seen and was incredibly capable for its price. I still have mine, and I still like it, mostly. I've had people on the subway ask me about it simply because they didn't know what it was :P.
I thought Gnome 3 was awesome and a gigantic step forward from Gnome 2. You could customize your entire environment, all in JavaScript. And yet almost nobody I know uses it. Unity may have gotten panned by internet critics, but at least plenty of people I know use it. Gnome 3? Maybe one person.
A disproportionate number of the things that I get really excited about--rather than just interested in--seem to fizzle out. Maybe it's just selection bias, of course. I imagine the contrast between my expectations and reality make things stand out more. But it does seem that the more enthusiastic I am about something the more likely it is to flop.
Right now, the technology that I'm really excited about in a very similar way is Rust. I hope it breaks this rather annoying trend! It's basically the only new programming language that I genuinely like and don't view as a waste of time and space. I honestly think it will take off, but that's what I thought about all the other examples too.
I guess this doesn't have much to do with the actual article. It's just a lament about my own misplaced enthusiasm. But it does feel better to have written about it :P.