What damage will powering down instead of shutting down do?

It's unlikely that the whole machine will die, but if there's anything still in file write buffers, you'll lose that data... and basically the machine probably won't know the difference between you powering it off and it shutting down due to a power cut.

Services won't get the chance to shut themselves down cleanly. For instance, if you're running a web server, when it gets asked to shut down it may well complete any existing requests (with a timeout) rather than the connection just going away.

The "file write buffer" doesn't just have to be the Windows buffer, either. Again taking a web server example, the logging might be buffered so it only writes to disk every 100 requests or something similar. A clean shutdown will flush this appropriately; a hard shutdown won't.

If you have online services, a clean shutdown may sign you out of them appropriately, instead of the service thinking you could just have network problems.

Basically think of anything a system might want to do when closing down in terms of either the on-board disks or connections to other systems (such as network connections) - all of those are going to be happier when shut down properly.

Jon Skeet has already made all the critical points from a software and OS standpoint.

I have just one small point to add that might possibly be of significance.

Shutting down the computer where the motherboard chip-set comfortably powers down the system is different from a sudden power cut to the system.

At the minimum this could damage your power supply unit (the thing usually behind the rear power-switch on your CPU box). In a worse case, the motherboard power regulator devices may breakdown. That is a motherboard replacement cue usually. And, there are worst cases.

If you have an emergency, at least get to a standby or shutdown state with a depressed power button -- takes not more than 3-5 seconds -- before you cut power.

There could be disk activity happening in the background that you're not aware of. You could end up with corrupted files. I'm not saying you will but it increases the chances.

Examples of things that could be happening even when you think the machine is idle:

• Defragmentation
• Windows Updates
• Other software updates e.g. virus definitions

I wouldn't expect it to do any more harm than pulling out a USB drive without ejecting it - usually fine but could do damage if it's quietly doing something important when you switch it off. I suppose it depends on how much you dislike reinstalling if it does break it :)