According to 2K Marin's Kent Hudson, there are a few core qualities an enemy character should embody in order for playing a game to be fun. Most important among these is predictability. An enemy can be difficult to beat, but the player needs to be able to learn its movements and adapt to its strategies. Otherwise, attacks are coming out of nowhere and there's no way for the player to anticipate and adapt.
Makes sense. If only the team at 2K Marin had realized this before creating the Big Sister.
"We painted ourselves into a corner," says Hudson. "We made an error of innocence."