Yes. A mob shouldn't kill itself voluntarily.
Yes. It should do meaningful actions.
- The way it acts around players?
Yes. Mobs shouldn't ignore players.
Yes. The mobs shouldn't lag the game much.
Yes. The mobs should drop items which are useful and otherwise unobtainable.
- The way it interacts with the world?
Yes. If such interaction makes sense.
Also how they look like and what sounds they make.
What mobs would people like to see in the game?
Some mobs which are both useful and challenging.
Should they be difficult to deal with or dumb and easy to kill?
It depends on how frequently they spawn. I prefer many weak mobs rather than few strong mobs because when there are a few strong mobs, it's easy all the time until you meet a mob and then you get frustrated.
Should they know where a player is at all times or should they look where they are and attack them?
The latter, if it won't make them too laggy.
If you lack the reality, go on a trip or find a job.