Physical assets that depreciate much faster than snow melts in the sahara and dividends that are function of difficulty. Have you looked at difficulty or network hashrate charts recently? Yesterday difficulty went up 14.5%  and  its currently projected to increase >18% in 11 days. That means dividends go down by the same amount, and your physical assets depreciate by about  as much, and this is even before the big onslaught; Asicminer based rigs start shipping this or next week, good for a total of 60-100PH (!). Antminer starts shipping its 28nm S3 next week. KnC starts shipping and probably deploying its 20nm this week. Heck, even BFL may finally begin deploying some Monarchs this week, and then there is my hunch that a 28nm Bitfury is just around the corner.