Kaisterkai, here's the lowdown:
The single biggest killer of electronic and electro-mechanical devices is thermal cycling, ie the heat-up/cool-down cycles that go with powering up and powering down. This places strain on solder joints, contacts, bearings, &c and this can shorten the life of components and connections. Every time a PC powers up current flows through a myriad minute connectors, which heat up and expand microscopically; when you power down, the connectors cool down. You get a cycle of expansions and contractions. Over time some tiny joints and connectors can develop a microscopic hairline crack but still remain connected, which mean the same current has to flow through an even tinier connection, which causes more heat (and expansion) and increases the odds of failure. Eventually the connection/joint fails completely, and your device develops a fault.
HDDs also struggle with power-on/power-off cycles, or more specifically heat-up/cool-down cycles. Cold drives powering up experience trmendous strain on bearings and small mechanical components, which are designed and manufactured to run optimally at their normal operating temps, ie they are made to be looser and slacker when cold so that when they reach operating temp the metal components expand to the correct tolerances.
It's to overcome thermal cycling problems that some sensitive components and devices have a "stand-by" function. CRTs are particularly prone to thermal cycling failures.
That said, modern components and HDDs are pretty reliable, and there's no real need to worry too much about this.
Still, all my PCs stay on 24/7 (unless being moved), particularly for the sake of the HDDs.
