An "expansion" of sorts to the original Metal Gear Solid Portable Ops, "Plus" delivers new content, new characters, and an all-around new experience.
First, the basics. Portable Ops Plus is a standalone game that does not require the original Portable Ops to play. Possessing save files from Portable Ops does, however, let you bring your old save files over and continue playing with all the characters and items that you unlocked in Portable Ops. So, for example, if you had assembled a highly skilled team and unlocked all the unique characters in the original Portable Ops, those characters would not be lost. Hideo Kojima, in interviews, said that he envisioned the original Portable Ops as multiplayer only and decided to put in a story later; Portable Ops Plus is more like this vision, with everything designed to supplement multiplayer play.
The main mode of play is "infinity mission", a sequence of missions that range from easy to extreme. The harder it is, the more likely you will find rare characters. The player moves from stage to stage, looking for exits to the next stage while collecting items and characters in a manner similar to the original Portable Ops. The main problem with this mode is that, except for a rarely found item (the Fulton Recovery System) there is no way to leave the mode except to beat it all the way through, which usually may take a good hour to an hour and a half. You cannot save and quit midway, either. Soldiers who are killed during the Infinity Mission mode can be recovered later as prisoners on another playthrough. However, what happens sometimes is that you will recover your character, or find a really good new character, and then later lose another character. The choice is "lose your new characters" or "lose your old character and have to come back for him later", a difficult one given the amount of work that has to go into each Infinity Mode.
The characters and their management has changed too. The main change is the addition of many character types from earlier Metal Gear games, and the addition of female versions of some of the soldiers in the original Portable Ops. However, the system of advancement has changed, as well. Characters now get experience for going on missions or being in a support unit (the spy, medical, and R+D parts of the unit). Experience raises LIFE and STAMINA scores, invaluable for surviving the long process of Infinity Mode. Furthermore, textbooks can be found that will raise a character's proficiency in a given field (for example, pistols, assault rifles, or setting traps). Formerly, a character's proficiency was unchangable. There are still limits though - a character has a maximum amount that each limit can be upgraded, so that you can't build a single "supersoldier" (though you can at least come close). New careers have been added to supplement the new gameplay, as well; with the emphasis on collection, the "Headhunter" and "Decoy" classes (among several others) have been added. The Headhunter can see an enemy character's stats, allowing you to judge if they are worthy of being captured. The Decoy makes all enemy characters stronger, which is bad if you have to fight them, but good if you want to capture them (as their strength means they will be better once they are on your side).
To accommodate these new characters, the maximum level has been raised from 100 to 200 soldiers. Furthermore, the number of "teams" - the units you send out on missions - has been changed from 4 (A through D) to8 (A through H). These teams are used for almost everything; in addition to their old uses, such as single player missions and multiplayer, the teams are also used for online trading (send a team online to offer up with set requirements for what you want in return and wait for a response from someone else's team) and AP Scan, a way to collect new soldiers by using wireless networks. In the old game, the player simply had to be near a wireless node - near enough for the PSP to detect it - and mash the "O" button until the soldier joined up. In Plus, the requirements for nearness are lessened (you rarely have to walk around looking to boost the signal strength) but in exchange you have to send out a team; the prospective new character will tell you what he or she is looking for (ranging from "I want someone with an S rank in some weapon" to "I want someone wearing a nice hat" to "I want a person who is a Doctor"). If you have what they want, they will join you; otherwise, you can go out, reassemble your team, and come back to that hotspot. Characters will not change, so you can always come back and collect him later. The key is to make one's team as diverse as possible, with a wide range on every single set of stats, so that all prospective enlistees will have something to look for. Furthermore, the characters are of much higher quality now from AP Scan mode (as opposed to before, when they had random stats that could easily be fairly low, all characters obtained from AP Scan are higher level than most characters collected from single-player mode).
Multiplayer is, of course, the basis of the game. The main infrastructure mode is standard online play, boosted by the many new characters, weapons, and maps introduced (many of which are similar to or copies of maps used in MGS3's multiplayer mode). Also introduced is the new "motion" system, a form of taunting that expands from the basic "salute" into a wide variety of poses, which can be expanded by finding and using "fashion magazines" in single player. These poses range from bowing to cheering to Revolver Ocelot's trademark gesture from MGS3 to a wide variety of "Justice Poses" used in various Japanese superhero shows. There is also now a "beginner's server" in multiplayer; going in will give you the beginner's status, which reduces a lot of the damage you take from headshots and explosions. After a certain number of kills, you will lose the status, showing your "graduation" to a regular player. Finally, one of the new game modes is "chatroom", which is a server with no fighting and no goals; it is, as indicated, a room for people to talk using their MPO characters as their avatars.
With this array of new content, MPO+ is almost an entirely new game, and is definitely more than I thought it would be. While the "infinity mode" gets fairly irritating, the wide variety of new content keeps me entertained throughout it.