In both Playstation Lunar games, enemies have special animations they cycle through during the beginning phase of a combat round, while the player is still inputing commands. Each enemy generally has at least two animations, while bosses often have many more. Each of these animations corresponds to an attack. For example, an enemy that is standing in a neutral pose might just use a basic physical attack. However, if that same enemy rears back threateningly or starts to glow, then it might cast a magic spell or use a special attack during the turn. In this way, the game tells the player exactly what every enemy is going to do during combat.
While giving the player this information sounds like it would make the game easier, it really doesn't. Both Lunar games are actually really hard. Even though the player knows what attacks the enemy will use, the enemies are still going to execute their attacks regardless, so the player's characters are going to suffer the same number of attacks either way. The difference is that the player can use this knowledge to approach a battle with more complex strategies.
In a typical battle system, the player has no way of knowing what the enemy will do, and has to react to situations blindly. When facing a group of six enemies, the player only has one strategy: kill all of the enemies as quickly as possible. However, if the player knows that two of those enemies are preparing to execute a dangerous special attack, new strategies open up. The player can choose to either focus on wiping out the entire group, or focus on disrupting the attacks of the two enemies. If the attacks are particularly dangerous, the player even has the choice to focus on defense. These added tactical options engage the player on a deeper level than most RPGs, vastly improving the player's experience.
A good example of Lunar's combat system in work comes from the boss battles of the game. Bosses have lots of attacks, and thus lots of different idling animations to broadcast those attacks. For example, when the final boss of Lunar: Silver Star Story Complete surrounds his outstretched hand with dark energy, he is preparing to use his Hell Wave attack, which hits the entire battlefield for a lot of damage. A smart player would have every party member defend that turn. Developers often give this kind of "force the party to defend" attack to boss monsters in RPGs, but it usually involves the boss wasting a turn broadcasting the attack with a charge action, effectively giving the player a free turn to beat on the boss. I think Lunar's solution is much more elegant.
This system also has the effect of rewarding the player for being cautious. In the early stages of a Lunar boss battle, the player doesn't know what attacks correspond with the enemy's animations. So, the player is encouraged to fight defensively for the first several turns while he learns what moves the boss uses. If the player orders his characters to fight recklessly, the boss might flatten them all with a big attack. So the system encourages the player to study his opponents and come up with tactics to deal with bosses.
The strategic combat created by monster tells is what I remember the Lunar series the best for. Few developers have tried to focus on making combat more tactical by giving the player more information. Yet, I think it is one of the best ways to do so.