These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Awesome Music: Let's play it safe and make a blanket statement: Every single song in Metal Gear Ac!d 2 is awesome. Many players rejoiced when they found out the iPod in Metal Gear Solid 4 could play back "The Essence Of Vince."
Complete Monster: In the second game, Wiseman instigated a horrific event known as the Praulia Massacre: Wiseman felt that the data about the Model 2 soldiers was insufficient, so he had technicians provoke said soldiers into going berserk, causing the deaths of 10,000 people. He then sent the Model 3 soldiers in to eliminate the Model 2 soldiers, knowing that eliminating their brethren hurt them. Besides this, Wiseman was also responsible for the death of Koppelthorne's wife, which also resulted in the events of what happened later on. In addition, he had Venus render Snake amnesiac in to help him obtain the Lucinda File with the evidence of the massacre, and had Venus execute him simply because he outlived his usefulness.
Game Breaker: The Metal Gear cards (which replicate the attacks of various different giant robots throughout the franchise) essentially kill all non-boss enemies in front of the player and can be acquired by buying/finding booster packs - the only downside being a gigantic amount of cost, but most players have no use for it when a room is cleared of enemies. Also, enemies cannot move past the bodies of of fallen character (if one of them is killed) which can allow the living character to get to safe location and use long range weaponry. Its only downside is its cost (25 - 23 or thereabouts if you upgrade it), which means you must wait two or three full enemy turns to move again.
The XM8 is even better, it is cheap, got a good hit rate and when used it sends a flurry of Standard Status Effects onto the enemy, and it gives a very good amount of Move Panels if you decide to use it as a Move Card, and it can be obtained right of the bat with a password.