Heat.  It shortens the life of your PC.  That's why so much attention is paid to cooling your system.  Fans draw air in and force it out of your PC, allowing it to flow over the chips and heat sinks in order to keep things cool.

Dust.  Unfortunately, the cooling fans also suck in dust and dirt. It clings to every electronic component in your PC and prevents air from flowing across the chips. Instead, the air flows up and over the dirt, allowing heat to build up in your PC. 

The photos on the right show a six-month-old Pentium 4 used in a home office.  Not much cooling going on in this PC, especially in that high end video card.  When the owner complained that his computer kept crashing, we cleaned it out and put it into a Cleane-Aire Computer Enclosure.  It hasn't crashed since.

You won't find much written about dust and dirt in PCs.  But, we've all known for many years what it can do to a computer.  Read the article by Larry Mingus, written in 1998.  He put the issue in perspective many years ago.

But, don't clean your filters too soon. Even with all the dirt built up on the filters below, air flow was not restricted.