All of my attempts at capturing cows, chickens, or sheep to corral and keep for convenient farming have been thwarted by the animals despawning. A great addition to the behavior of the animal mobs would be if they became "domesticated" (stop trying to jump over the fences) once fenced in and if they became persistent. Possibly add in structure requirements to keep it balanced (ex. 1 chicken coop supports X chickens, 1 barn supports X cows, etc.)
This idea is really cool, considering that I dont like animals despawning, but for the barns and 'living' areas for animals should only have a limit. like what Redstone_Fanatic said that "there can only be 16 passive mobs, otherwise there will be problems", the limit would stop it from having problems like 1 cow + 2 chicken whatever. Then it wouldnt have 'problems'.
No but seriously, only 16 passive mobs can be on a map at the same time, otherwise there are problems.
WHAT?!
Danger zone.
Checking already gives you 25% addiction.