Reasons why it's never a good idea to position your PC on the floor....
For some reason, a lot of my colleagues at work and even some of my friends in their homes prefer to position their PCs on the floor, leaving only their monitors, keyboards and mice on their tables. Frankly, I don't think this is such a good idea. True, the jumble of cables coming from the rear of a PC isn't exactly aesthetically pleasing, but if one values reliability, durability and ease of use above aesthetics, it is always preferable to position a PC in an elevated position such as on a table, and not to leave it on the floor.
Modern PCs have plenty of components which generate a lot of heat such as the CPU, the northbridge chip, power supply and video card among others. To keep these components cool, a PC typically has at least two fans, often more, and are configured to suck cool air from the front of the PC, through the chassis and out through the power supply and/or an exhaust fan at the rear. A PC on the floor tends to suck up the dirt, dust and grime on the floor (even animal hair if you have pets), clogging up heatsinks and coating electronic components, making them less effective in releasing heat to the air. This results in higher operating temperatures which could leading to premature component failure. It's even worse if you sweep in front of the PC, or if your floor is carpeted, as the PC acts like a vacuum cleaner sucking all the dust and dirt in front of it.
Frequent exposure to dust can also cause the optical drive to malfunction as the laser read/write lens can get coated with dust;
In office settings, cleaning crews typically scrub and mop floors during the weekends. It's not unusual for these crews to unwittingly splash water and detergent on PCs located on the floor. Most vulnerable to water splashes are the ports at the rear. Water can also get in through the various air inlets. Moisture can lead to corroded contacts and/or short circuits. The chassis could also rust;
PCs on floors are vulnerable to various small animals/insects. Mice can chew on the various cables, or urinate on the rear ports. Cockroaches may crawl in the small holes of the chassis and lay their eggs or other secretions on the various components. These can be corrosive and eventually lead to component failure;
PCs on floors tend to be more exposed to inadvertent kicks from people sitting in front of them. A kick while the hard drive is reading/writing could result in the read/write head smashing into the hard disk recording surface, possible leading to a hard disk crash, bad sectors and/or loss/corruption of data;
From a usability point of view, it's not very practical to always bend down just to plug in a USB drive or insert a disc in the drive, when you can just reach over if the PC is on your desk;
It also helps if the hard disk access lights are visible. That way you can determine at a glance if your PC has already frozen, or if it's just reading from the drive.
If you really prefer to set your PC on the floor, get one of those stands to elevate the chassis at least several inches from floor level. This would mitigate some of the hazards mentioned above. If your PC's chassis is one of the floor-standing types that are meant to be placed on the floor, keep the area around the PC free from dust or pests.
Lastly, if the jumble of wires from the back of your PC bothers you, invest in wireless keyboard mouse/combo. Nowadays they're dirt cheap. Pardon the pun.