Fans are the ‘Achilles Heel’ of Your PC

"In a particulate-rich environment, the stuff (dust, dirt, sand, soot, etc.) builds up on your fan.  The fan, in turn, slows down and needs to run longer -- and push harder -- to get the same effect. Meanwhile, your components are covered in a kind of dust blanket (which can be another whole set of conductivity problem in its own right), keeping them insulated and causing your fan need to work even harder. This is where things get hairy.

... Going fanless means longer lifespans and increased reliability."

Excellent article from Logic Supply's John Donoghue.