My suggestion has been to have tremor proc one second once it's initially dropped so good shamans can outplay teams by dropping it right before they get feared, but then after that proc it will proc every 3 seconds so it's initially the same vs bad shamans.
That's how it was before the bug/nerf/WotLK; Priest coming to Fear bomb? Drop well-timed Tremor=>Feared=>Cleansed. I did like it that way because I did still get Feared sometimes, but it was probably a bit over the top on how many it broke. Still, a good Fear usually makes the game though.