I find that Inactivity is the biggest killer.

The person you're talking to IC disappears? At least you can go off and talk to other characters. The GM and >3 people stop posting? Entire scenes, interactions and conversations are killed. I, personally, start wondering "Well, what's the point in talking to a character if it's a coin toss whether they respond or not?" I end up waiting a week for a single response only to find more people have vanished and I'm left sitting there thinking "Well, fuck."