What kind of elements keep users coming back to a game - particularly a multiplayer-based (or simply incorporated) game?

I guess I'm just not a very good sociologist and I'm wandering what you guys think are addictive (for lack of a better term) elements of games?

Grinding, grinding, grinding. Sad truth but I think it is all about that. Speaking about social game the principle is simple. Accumulate 10 woods you can build an house. Good now accumulate 100 woods and you can build a bigger house. This can go like that for a very very long time. Just give players then envy to build houses (goal) and then they will give lot of value to woods (resource).

Looking at the friend system that they all implement, I think it is very smart business wise:

"You are gifted 10 woods if you invite a friend into the game."
So basically your players will do all the marketing for you.

"Friends gift woods to each other every day they connect."
Your players will keep going back because they are socially involved. If you are lucky they will get addicted and pay some cash to get those might gems.

There need to be either

1. new experiences. There should be more things for the player to try. This is especially effective if they have seen other players make effective use of the content or activity in question.
   
2. areas of improvement. Things the player has already done, but would want to do better. Particularly effective if the player can show off their prowess.
   

There should also be social opportunities. Players will often enjoy playing a shooter or driving game in person on a console so that they can enjoy taunting each other and showing off in person. Good chat systems, leader boards, etc. can help take on this role online.

What kind of elements keep users coming back to a game - particularly a multiplayer-based (or simply incorporated) game?

Ranking tables. Ever since the era of coin-ops and Space Invaders and even before that games had ranking tables to show the players just how good their skills are in comparison to others. People play games to overcome challenges and feel a sense of accomplishment. Ranking tables can be a means to show the player that there's someone better then them thus driving the player towards improving their skill (in other words replaying the game).

It was sort of touched on before but I'll say that you need to provide a sense of progression. Personally I get tired of a game when I don't feel as though I'm moving forward in some way. If the game is highly competitive then I'd think you would want to try and help match up opponents by their skill levels as best as possible.

It's the same angle that drive MMO's, the value of play. You either need to be able to constantly give players rewards for their play or introduce something that compels them to continue, like a strong narrative.

I though MAG bruched up on an interesting idea but fell short with execution which was the 3 separate factions. Where they fell short was that they could have used this as a way too introduce a narrative into the multiplayer world. They could have taken a set time period (month or so) to evaluate the standingd of each faction and introduce new elements to the game based on the outcome. Keeps people interested.

Games like Minecraft and Terraria and LittleBigPlanet are leveraging User to create the content and I think it's something that a lot of multiplayer games are missing out on.

I think the most interesting part is to explore a world. If you haven´t yet explored the whole world, you would probably want to finish your journey...
Furthermore it´s often friends that bring you back

I'm not a fan of the skinner box mechanic -- where by grinding away, you'll randomly be given some cheese.

I am however hooked on Battlefield 3, which uses a similar carrot-on-a-stick mechanic, of having a huge array of items to unlock. Each time I log on, I can see "ooh, 3 more M16 kills and I get a new scope". I'm always given sight of a new carrot, and when I get it (which is a new toy to play with, a new way to experience the game), they immediately dangle another carrot in it's place.
Besides the above system of character progression, where I'm made to feel like I'm achieving something when I play, it's simply a well-balanced, extremely refined game, which happens to be a lot of fun. There's always little moments of satisfying novelty, like only using a pistol yet beating a helicopter-gunship, or leaping from a burning plane into a dense firefight, etc... Lastly, it's voice-chat integration and squad system makes it a great way to hang out with friends.

I'm also hooked on Starcraft 2, due as above, to it being an extremely well-balanced, extremely refined game, with an incredible depth. Aside from learning the mechanics of how to play (which cover 3 skill-sets; macro, micro and multitasking) you've also got to learn how to read and execute the major strategies (which are part of an ever evolving meta-game discovered by all players). SC2's success for me comes from it's well-implemented online matchmaking system, which segregates players into skill-groups ranging all the way from "I just bought this game, what's a RTS?" to "I'm so pro, I make $300,000p/a in endorsement contracts and am invited to $100,000 tournaments monthly", which gives everyone a roughly 50/50 chance of winning a game, and a good record of their actual skill improvements over time.

Just noticed I have been rambling on my last post, sorry about that.

Another aspect related to MMO is the fact that the items, resources, levels or whatever you acquire during the game is permanently stored on a server. It is not stored on your local machine and will stays as long as the game lives. I think this gives a feel of reality (as opposed to virtual things that can be erased) and makes collectibles much more valuable to the player.