Well, to answer the first question, I generally use the chaos theory when playing -- affect as many people as possible WITHOUT resorting to game-stalling cards.
That being said, I usually launch attacks "to get things going" before I establish a solid defense -- impatience is one of my flaws.
As for deck types, anything that is creature heavy. Even monoblue decks -- at best I run 4 assorted counterspells (again, too impatient to use them properly -- or worse, off getting food when something worth countering is cast). So I guess beatdown decks are my preference. The only combo-type deck I have involves Windfall and Megrim and all the rest of the global discard. People don't really like that one.
I like "party" cards, such as Wheel of Fortune, Veteran Explorer, Invigorate, and Noble Benefactor. My logic being the more cards people have in play, the more interesting the game gets.
Other fun cards are those that speed up game play, either through speeding library depletion (Howling Mine), increasing the mana supply (Mana Flare), or inflicting upkeep-based damage (Copper Tablet, Urborg Stalker, Forsaken Wastes).
That being said, I try to build decks with some synergy. Urborg Stalker and Ascendant Evincar both work well in monoblack (permanent, at least) decks. Add red for multiplayer burn (Sizzle, Flame Rift), and you have the basis for a mean multiplayer deck.
Diplomacy for me is very much a give and take issue, and can quickly turn into a game of king-making (I die next turn, but I will see to it that person A dies, too!).
Reset buttons that your deck cannot take advantage of slow things down, and should be shunned. Wrath of God screws everyone, but if your deck has creatures that come back (Avenging Angel, Ivory Gargoyle) then its okay, as you can wing your way to victory on your resurrected creatures. You get the picture.
Life-gaining on the whole is acceptable, but when done in obscene amounts (like Congregate in an infinite squirrel deck), it only results in your being targetted by everyone. If you do run considerable life-gaining, also run cards that use life as a casting or upkeep requirement (Cleansing or Glacial Chasm), in order to not be as large a target.
Cards with ACC really come into thier own, as the conditions can be met fairly easily. Especially the instants.
Don't be afraid to play the cards with large casting costs, as multiplayer games take longer than duels (thankfully), so you can actually hardcast that Leviathan AND attack once or twice with it.
In some circles, decking yourself can be a real possibility. Try to include some form of graveyard recursion (Feldon's Cane, Gaea's Blessing, Rancor) in your deck.
Monocolor decks are fun and powerful, but two or three color decks prove far more versatile. In addition, while 4 of a card ensures consistency, try running single or double copies instead. This allows for more variety and encourages creativity. Who knows, you might even discover good dueling "tech" as a result.
That about sums it up. My feet are numb from three days in the mud and rain. My unit's been released, and I am off to bed.
Goodnight.