How many great ideas have we had that fall away because they didn't work or are unsustainable? How many have you had personally? Here's a couple of mine:
Moodle Testing. Turns out it is a lot of work learning the coding, making the tests, and chasing down kids when they don't take them. I also get the sense that there are better test mechanisms out there.
Forums. It worked great the first year when I emphasized it and gave students credit for it. I guess I thought the next year it would simply take hold organically without credit or emphasis this year. It didn't. Complicating matters, I didn't know whether it'd be best to keep the old posts up or have kids start fresh. I suppose it depends upon my goals, which I did not articulate.
Those are two, and I'm sure there's more. Do you have any ideas in the #mathgraveyard you'd like to share?