Before I start this article, I would like to say something about my experiences with magic. I enjoy casual magic, the kind of games where you DON'T spend your first few turns laying out a general strategy of "I got my combo, you lose". I also enjoy multiplayer magic, and I believe that is where the true strategists can thrive. Casual multiplayer magic is not bound by restrictions and rules and errata like the tournament scene, but by ingenuity and survival. Common sense outweighs combos by leaps and bounds, as you try to find a way to win and seem non-threatening to the other players at the same time. In my opinion, this is the ultimate form of magic.
Now that I have said that, it's time to talk about what should and shouldn't be in a casual multiplayer deck. One point to keep in mind is that this list is not meant to be a 'banned/restricted' list, but more of a guide to having fun in a multiplayer environment. But there are some cards, just because of their abilities, should probably be left out of a multiplayer deck. Power cards are first on my mind. If you play a first turn Lotus, it says that you probably have some sort of trick up your sleeve, which makes you a target. Also, discard decks are the bane of multiplayer games, since no one likes discarding and that type of deck probably won't last more than five or six rounds, maybe less. Another card best left out of the deck are the ones that produce adverse global effects, i.e. Winter Orb, Forsaken Wastes, and the like. Nobody likes being hamstrung in a casual game, and that player's early exodus should be expected. Burn and counters should also be thinned out, but not excluded. Finally, spells that punish a player for using a color should be unquestionably 'banned', no explanation needed.
What SHOULD go into a casual multiplayer deck is often a matter of personal preference. Some like to use spells that normally would be too expensive in a 'kill or be killed' environment, others include a stack of utility spells to deal with those cards that shouldn't even have seen the table. Some like to include permanents that reward players, like Howling Mine or Horn of Plenty. Personally, I like to build decks that fit into the 'rogue' category. This is the main reason why I like casual games. Most of my decks don't see a third or fourth game, but as far as I'm concerned, they were good. Granted, some of my decks were doomed from inception, some couldn't hold my interest long enough, and some just didn't work the way I expected. But with the bad comes the good, and once in awhile I'll make a deck that flourishes in multiplayer and still holds its own in duels. This is the reason why I enjoy building rogue decks. I like hearing 'where did you come up with that' or 'why didn't I think of that'. Now I don't want to come off like I'm saying that I was the first to think of a type of deck, but I like putting together combos that I haven't played against yet. This is why multiplayer magic is so appealing to me. It gives me a few turns just to play land (and nothing else), and a chance to see what's going to hit the table. Sometimes a good idea can turn really bad really quick, and in a long multi game (in the hours range), it's best not to be the first one out, since you could be waiting around for awhile.
In my opinion, diplomacy is the thread that holds multiplayer magic together. To make and break alliances throughout the game gives the game life, and that is very important in long games. You don't want people that get bored after the game has turned into a political tug-of-war. That is usually when it gets interesting. Sometimes the cards on the table interact in strange ways, which could also be fun if played right. But you can't forget the one or two players who didn't take the opportunity to mulligan when thay should have (this is a very common mistake for beginners). Struggling players should be allowed to 'catch up' to the rest, since it's not fun to get beat down without the possibility of defense. In some games I've played in the standing rule was: if a player has no creatures in play AND has no way to prevent, reflect, or redirect any combat damage directed towards him or her, then that player should be immune from attacks, directed spells, and the like, and also should not have any creatures or defensive spells countered until they can 'hold their own'. Also, if a player only has one creature, land, or non-detrimental artifact or enchantment, it should not be destroyed. Multiplayer magic can be more sporting than any other type of magic, and this should be reflected on the games you play. I like to form alliances with those guys, since it definately helps to keep the game interesting for everybody.
Finally, multiplayer magic is meant for experimental decks. Creativity is very important if you want to build a deck that is actually enjoyable to play. Good candidates for multiplayer decks are ones that you liked, but didn't win much in duels. If a deck cannot win one-to-one, that dosen't mean it is a bad deck, just a bad dueling deck. Creature-heavy decks usually do very well in multiplayer games. I have seen simple Thallid decks kill many, many people at the same time in multi, but I also have never seen one legitimately win a duel. And remember, fun is of the utmost importance in multi games, and no one should be singled out without a good reason.
So brush off those cards you've never used, throw in some land, and invite all your friends to play a game where the last thing to think about is winning. Who knows, you might even have fun (gasp!).