Leaving the Computer on.

Overheating would only be an issue if the cooling provisions weren't sufficient. Liquidshadow is correct, the life of a component can be shortened, but similarly constant turning off/turning on can increase the rate of electron migration, so it's six of one and half a dozen of the other.

Bear in mind that computers are almost always upgraded long before the parts wear out, whether it's left on 24/7 or not.
 
you could set it to turn off monitor, HDD, and some other stuff to prolong life when not in use for a few hours.
 
Unless there is a good reason you want to leave it on - turn it off!

It is a waste of energy. Whether you pay energy bills or not, it is a waste. I'm not exactly a tree hugger but this is a bit too much.

Besides that, computers are meant to be turned off. It clears the RAM. It would probably run a bit better if you just turned it off at night and back on in the morning.
 
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