New Game Plus is probably one of the coolest, most obvious, and underused features in video games today. There is no better way to get me to immediately replay your game than to give me every single item, weapon, magic, and point of experience that I finished the game with at the start of the my next playthrough. Yes, it makes everything Win Button easy, but it is so very satisfying returning to the boss the gave you so much trouble the first time and one-shotting him into oblivion. New Game Plus should be a required feature of every RPG and adventure game.
For the unaware, New Game Plus means starting the game over but loading your characters from when you last beat it. You generally retain most non-story items and weapons, and keep your existing level and stats. It's generally a nice reward for conquering a game, but as we'll see, can also be used for a variety of reasons.
While many sources point back to the original Legend of Zelda as the first to use the then unamed New Game Plus, it wasn't really what someone would expect a New Game Plus option to play out. Starting a new game from a finished save unlocked the Second Quest, a more difficult version of the game with different enemy placement and dungeon layout. For this to be the true source of the trope, Link would have needed to at least retain the weapons and tools he collected during the first playthrough. While someone might complain, "that would totally break the game!" Well, that's the point of New Game Plus.
We really begin with Chrono Trigger, my favorite game of all time and also the best example of the New Game Plus feature. After you beat the game and watch the regular ending, you can then select from either New Game or New Game +. When selecting New Game Plus, you also pick a saved game to "copy" your band of characters from, and you're off to save history again. Only this time, you have everything you had last time around and are just as strong. Crono will have his Rainbow Sword and Luminaire spell to utterly destroy every enemy in his path. Early bosses go down with just a few strokes of the blade and you'll laugh at how simple it is now. Difficulty will scale up just a bit as you go, but it's never any more difficult than the last time around.
The real bonus to this mode is the ability to view all the game's endings. Chrono Trigger is famous for not only being awesome, but also featuring about 15 endings, most which can only be unlocked while playing New Game Plus. The game is silently broken up into chapters, and challenging and defeating the final boss during different chapters let's you see a different ending. So not only can you now take on Lavos at any time, but you can begin to decimate him every round.
Chrono Trigger's endings range from silly to story enhancing, but my favorite is the developer's ending. To unlock this, you have to beat the game as early as possible, while you still only have two characters available to you. This is a decent challenge because you just beat the game with three characters, and now you have to beat it with just two (and one is Marle, who is kind of weak) without gaining any more experience. Of course, you could wimp out and start another New Game Plus after beating the game a second time, but it's really fun to take Lavos on right away again. The ending is definitely worth it too, as it's playable and every developer who made the game can be spoken to for personal messages.
Since Chrono Trigger, there have been a ton of games in a huge variety of genres that have implemented New Game Plus in some way, shape, or form. Here are some notable ones that I've played:
The first Mass Effect was a recent example that required you to play through the game at least three times to get all the achievements, and for one in particular, you had to play a New Game Plus at least once (not that I'm complaining, I beat Mass Effect six times). The hardest difficulty available when first playing the game was Veteran, but after beating that, you unlocked Hardcore, and after that, Insanity. The difficulties available were shared across all characters, so you didn't have to keep playing as the same old soldier to receive the Insanity achievement, but if you wanted the Level 60 achievement, you pretty much had to play through the game three times. I received that particular achievement about 80% of the way through my third playthrough (second New Game Plus), but it was well worth it for me and very enjoyable.
While not a true New Game Plus, I was affected by Final Fantasy X's new game bonus which let you see what Al Bheds were saying at the beginning of the game. Essentially, there's a different language spoken in the game which is a one-to-one letter conversion from English, and throughout the game you could find Al Bhed primers that converted one letter to you, so whenever you saw the subtitles for that language, it would be partially translated. For example, "first hour" would be "vencd ruin" in Al Bhed, but if you have collected the N -> R primer, you would see it as "veRcd ruiR". With enough letters translated, it was easy to tell what they were saying. The thing is, you're not supposed to know what they're saying the first time you play it, but since I was playing on my friend's PS2 my first time... it was too obvious.
Dead Rising is a rather notorious example of a game that forces you to play a New Game Plus multiple times to actually beat the game. While this doesn't make sense in the context of what we've talked about so far, Dead Rising actually let's you start the game over at any time while carrying over skills you've obtained so far. While not impossible to beat the game without doing this, most gamers will need to (and several times at that), to actually beat it.
I recently beat playthrough 2 of Borderlands, which is almost exactly what it sounds. After you beat the game the first time, you restart with all your money, guns, and skills from the first playthrough, but the game also scales up the enemies to match your new starting level. This means the second time around isn't a walk in the park, but actually a more difficult romp across the wasteland. I thorougly enjoyed playthrough 2 and it really made the Borderlands experience for me. I'm currently working on playthrough 2.5, which is another basterdization of the New Game Plus. You don't actually start a new game, but all the quests you didn't complete in playthrough 2 are bumped up to around the max level of 50. So not only are the enemies and encounters very difficult, but the loot it extraordinarily awesome.
In my opinion, there are a lot of games that just do New Game Plus wrong, or games that are lacking it altogether that really deserve to have it. Many developers only go halfway, and only give you some items back, or reset your level, or just do random stupid things to piss off gamers.
For example, both Suikoden IV and Suikoden V transfer over all the items in your inventory, but not what the characters had equipped. What's the point? I don't keep my best stuff in storage, it's strapped on my Stars of Destiny for the final battle! At least the game lets you run faster during the replay, but that should have been available the first time around. Same with reading text faster; if your game has the ability to let me haul through the text at a speed I'm comfortable with, make it available to me the first time around!
Plants vs. Zombies also features a New Game Plus of sorts where you get to keep all the seed types you received the first time, but the game gets to pick three of the seeds for you per battle, basically relying on the A.I. to pick something useful. If I wanted, I could impose that restriction on myself.
Persona 4's New Game Plus resets your level, rendering all the grinding you did the first time around useless. If that wasn't obvious already.
When done right, New Game Plus is an awesome addition to just about any game. When done wrong, however, it's only a setup for disappointment. Imagine beating a decently hard game that you'd like to play through again for the story, but you don't want the challenge, but when you select New Game Plus, the game resets you at level zero because it thinks it knows what is best. Sure you do, you are the game the developers carefully crafted, and you're the game I'm immediately selling.
I'm a big fan of the feature, and think it would honestly generate a lot of replayability for relatively little amount of work. Original buyers keep the game longer and the used price is driven down at a slower rate, thus selling more more new copies. Seems obvious to me from this side of the fence. Just do it right, Chrono Trigger and Borderlands are the two best examples in my book of either making the game extremely easy (but awesome), or properly scaling the difficulty up to make it feel like a brand new game.