The thing that ultimately killed fiefs for me was its abstractness. I wanted to come up with a fun, relatable metaphor, but I think very, very hard about the true nature of things and an incomplete facade wasn't going to cut it. But when it comes down to it, I built fiefs around the idea of people connecting to the web and that didn't feel like a setting I was (at the time, or right now) interested in pursuing.
That is, I didn't want to make it a game like Uplink where the theme is hacking, or something computer-y, despite making a game that is by necessity computer-y. I should probably embrace it but I'm feeling not-too-computer-y these days. Maybe eventually.
I am absolutely still working on games in this design-space, on MMOs where players explore mechanically strange places. And, hopefully, affect one another in interesting ways. In fiefs you could see the footprints of other players' passage, recognize in part where they'd been. That's fascinating to me, and something I want to focus more on in the days to come. In the games to come.