Eventually you will experience one of the following problems related to the heat created in that space:
Power supply failure, Power fails to stay on more tha 10-15 seconds due to over heating again and again over time.
Motherboard solder failure, the slim doesn't have the YLOD it's light just goes out and the system will not boot up.
Game freezes, loading problems with your bigger open world or more graphically advanced games, again heat related sometimes it can cause the lasers in the blu ray drives to start going bad earlier than normal.
Not a huge deal now because parts, repairs, and replacements are available fairly cheap. It's not that hard to avoid problems and prolong the life of electronics.