The timesink.
When you factor in queue times, waiting for replacements who rage quit an instance halfway through, I'm seeing an average of easily 3 hours / day dedicated to just trying to land that days achievement points.
Hardly reasonable by any measure, frankly.
Lowering the heroic difficulty isn't really a satisfactory answer to the problem, but there most certainly is a glaring issue with how much time needs to be sunk into the game currently to progress. Nor is waiting a raid tier. The entire experience is rather frustrating.