An ex girlfriend of mine kept her computer right on the floor and when we moved from it hardwood floors to carpet her computer literally would not start while sitting on the carpet.
Computer on floor bad.
My pergo covered kitchen floor which has withstood every form of punishment two children can exact showed a small divot where the laptop landed.
For example cigarette smoke and pet fur will exacerbate those problems and putting your desktop pc on the floor will ensure more of that dust hair and debris will get sucked into the intake.
Ultrabook 1 floor 0.
I slid a piece of cardboard under there and it was fine.
On the floor keeping your pc on the floor is widely considered the worst place due to the amount of dust and pet hair that can get inside your tower especially on any type of solid flooring.
Not necessarily bad but not the best either.
If you have bottom intakes then you need to ensure you have enough clearance for air to get to those intakes.
If you don t have a choice you ll want to buy some compressed air and regularly blow out inside your computer.
Of course if you have short carpet on your floor the pc may be able to sit on that carpet with plenty of airflow underneath.