The most important things a PC can have are:
1) Reliable Power
2) Quiet operation
3) Sufficient automated backup capabilities
After that, I get to actual PC/3D performance.
Reliable Power is #1, because everything else hinges on the PSU. I'm not a fan of building boxes with 21 Gigawatt PSUs like most poeple seem to today: I actually build boxes with the correct power supply rating: usually not in excess of 450 watts. Occasionally a 550 watt makes it into a box if it's SLI, or has a bunch of SATAII drives inside.
In my view, noise is the biggest issue of all in the computing world of today. That's why I am building almost exclusively passively cooled PCs these days: passive motherboards, passive CPU coolers, passive graphics cards. 120mm case fans at low rpms, and 120mm PSU fans with speed controllers.
The boxes I build are typically almost silent, but have little or no overhead for overclocking. That's just fine, as my clients aren't interested in overclocked machines.
My most recent build is for a guy who does some CAD/design work, uses Microsoft Flight Simulator over New Zealand, in his LS1 glider with the 10 metre mesh for the whole of the country. The nice thing is though, it's driving dual 24" Samsung 244T widescreen LCDs.
Automated backups are taken care of by removable hard drive bays, and giant, tiny-cache PATA drives.