Retreating mobs are by far the most annoying. More so when they retreat and bring back multiple friends instead of just one to help them. Anything that automatically engages you shouldn't flee in fear when they end up losing. A mob that we attack and engage has more reason to flee in fear than something/someone that attacked us first.
Annoying for an NPC would be a vendor walking away while I am still buying/selling stuff to them.
A mob that just stands there and lets you whack on it has a name in most MMO's: Target Dummy
Mob behavior in MMO's is something that hasn't been explored to it's fullest potential in over 20yrs. It's time to move beyond path walking, low health running, meat sacks in most MMO's.
Dynamic Mob behavior models need to be implemented on spawn. Not every mob needs to be a special snowflake...but random mobs should have attributes that make them different. Maybe some mobs run and get friends? Maybe some mobs are afraid of fire? Maybe some mobs are merchants and have extra gold? Maybe some mobs spawn with multiple attributes and are renowned enough to have a name (random boss)? The possibilities are endless...and have RARELY if ever been explored in an MMO.
Dare to be different.