* The subject matter. I think the Lost 500 was more easily comprehensible as a finite subject, with well-understood genre possibilities. Never mind that we ended up choosing several of them; we had enough history with various forms of sci-fi that we could quickly come up with things to write. LER, on the other hand, didn't suggest any obvious genre hooks to me, forcing me to rely on my own creativity (the horror).
* The longer cycle. I think I started running out of steam towards the end. Three letters per turn worked fine for the Lost 500.
* LiveJournal. The Lost 500 had a discussion page within the wiki. It was crude and clumsy, but it was easy: Immediately after writing a new entry, I could pop over there and add a line or two. LER, on the other hand, had this LiveJournal group dealie. If I wanted to talk about something, there were a few hurdles: I needed to come to a different site, perhaps log in again, and weigh whether I thought the subject was "important" enough to start a new thread. Minor matters, and perhaps unique to me, but they added up to a moderate inhibition to discussing posts. And that seemed to take away some of the collaborative/friendly atmosphere, at least for me. I think the LiveJournal community was a good idea, but I think it ended up being counterproductive for me.
I'd love to hear other people's thoughts, even (or especially) if they take issue with my basic "LER didn't work as well as the Lost 500" premise.