1.
Some kind of mosquito proof screening where the water comes in. Most commercial barrels have this.
2.
A safe top design that prevents a pet or child from sitting on top and falling in and drowning. Most commercial barrels have small holes on top that lets water in easily, but not large objects. This is why it is unsafe to use an open trashcan to collect rainwater.
3.
Overflow system. Where water can flow away when the barrel fills up, to prevent flooding your foundation.
4.
Spigot and/or drain. So that you can get the water out.
Those are my votes.
Check out the
Rain Barrel Guide!