I've recently seen an uptick in Commander circles about a handful of topics related to game play and what is generally seen as appropriate or not. Since I see myself as the final arbiter of all things fair and just (I'm still working on convincing those around me of this), I thought I would offer my opinion as the final decision on these topics so the Commander community can move on to more pressing issues, like cards that interact with Divine Visitation.
Do: Play Infect
If you want to run a dedicated Infect deck, go to town! Decks with a handful of infect creatures and ways to give others infect are generally fun and add a level of tension to a game. This is coming from someone who doesn't play a ton against infect decks, so maybe I would change my tune if I was playing against them all the time. Although, there are only so many ways to build a dedicated Infect deck, so I would think that if you have a group you regularly play with, they aren't going to be building what is essentially the same deck.
Infect decks offer some spice to your play group. After the first couple of games, everyone knows what is coming when the commander is revealed, so the surprise aspect won't carry those decks far. Give it a shot and enjoy!
Don't: Play Triumph of the Hordes (alone)
Triumph of the Hordes is not an Infect deck. It doesn't create an Infect deck. Playing it in a deck with infect creatures, proliferate options, and several creatures that don't have infect makes good sense. It is a standard part of most Infect decks. When you are playing creatures with infect and some without, your opponents should be expecting Triumph of the Hordes.
When you put it in a deck without any other infect cards, it is the equivalent of an over-sized, undercosted Overrun. It is simply a "you lose the game" card. Playing it doesn't make you smart, it makes you, "that guy." Triumph of the Hordes, much like Expropriate, is a card that says that you built a deck and couldn't figure out how to make it win, so you gave up and added it to the mix. There is nothing fun or innovative about the card.
Now, if you can play it when an opponent is attacking another opponent, then I change my mind and tip my hat to you!
Do: Play targeted land destruction
Lands in Commander tend to get a free pass. Everyone needs lands to play their cards. However there are some lands that are just too good to be left alone. Lands that do things other than make mana are already powerful. Not destroying them is a serious mistake. Kessig Wolf Run, Maze of Ith, and Alchemist's Refuge are just three lands that you should feel comfortable wiping off the battlefield. There is also that group of lands that make far too much mana. Gaea's Cradle, Nykthos, Shrine to Nyx, and Cabal Coffers are all worthy of destruction. Packing ways to get rid of those cards just makes sense.
Don't: Play basic land destruction
I've played against Armageddon and Jokulhaups. I have "friends" who have built decks designed to replay these cards again and again while they rely on artifact mana to play creatures and spells to kill everyone off.
These decks are sheer misery. You draw cards and attempt to rebuild your mana base, usually with no lands in hand or only one. You wouldn't keep a one-land hand because it makes the game miserable, but that is what is happening when you destroy all the lands. I know some people suggest that you concede and save yourself the misery. They are probably right, but I have played in more than one game where the land destruction player destroyed all the lands, but wasn't able to deal with every creature on the board attacking them every turn. I have helped destroy the defenses of that player and taken them out. It hasn't happened often, but it has happened. So I won't give up and reward that level of garbage play.
Any player running this level of land destruction knows that they are creating a terrible play environment for the others in that game. We are trying to create fun games of Commander where cool things happen. The likelihood of cool things happening when no one has lands goes way down. I can't believe I even have to tell people not to do this, but here we are.
One caveat: if you can win the following turn by destroying all the lands, fine. I'll even go as far as saying that if you can eliminate one opponent in each of your following turns, then fine. However, you are going to get your mass land destruction card when you can't win the next turn, and eventually you are going to play it. Better to just scrap the deck and come up with something everyone will enjoy playing against.
Do: Play cool multi-card combos
Many of my deck ideas start with an idea that putting card A and card B together would be awesome. Then I realize that they don't interact the way that I thought or they need a particular board state to truly be amazing. Then I'm looking at one or two other cards to set up that board state. Pretty soon I realize that I'm looking at a five card combo just to get an effect that is fun, but doesn't even win the game. At this point I scrap it and find other options.
Then there are the other players who look at it and try to make it work. They dig deep and find other cards that do what is needed for less mana or at instant speed so you don't need all the cards. They work out ways to find the cards they need and produce these multi-card combos that win the game! This is awesome! The first time it happens you marvel at how the whole thing works and everyone talks about how it could be tuned to be a little better or laughs at how crazy the whole thing is! The next time it starts to happen, everyone is watching to see how close it is to happening. Generally, someone has a way to dismantle it, but even if they don't, we can all see it coming and ready ourselves. It makes for some crazy fun!
Don't: Play 2-card, I win combos
The dark side of the Rube Goldberg machine, is the machine gun. Someone gets two cards that when played together win the game. They play them out and ask if anyone has a counterspell. If no one does, the game simply ends.
There is no thought or skill involved in getting these combos together. No one in the group marvels at the combo or thinks it is impressive. No one wonders at the weird twisted way your mind works. The first time someone sees the combo, a little light goes off and they recognize that they have lost. Every time after that, they recognize that the only way to stop it is to have a counter at the exact moment when it is needed, and that rarely happens. Multi-card combos add joy to a game. Two-card, I win combos suck the fun right out of a game.
Do: Play strong decks that guide you into winning plays
If your deck is built to win a certain way, players soon learn that and can generally see how close you are to winning, based on what your deck is trying to do. Obviously some cards can alter the path of victory for your deck, and other cards jump you forward along that path, but your deck is trying to win along that axis. Whether the deck is strong or a little more durdle, you are moving along that path. When I play my Vorel of the Hull Clade deck, players generally know that I intend to win with creatures that are fat with +1/+1 counters, because that is what I'm trying to do with Vorel. The deck is built to win on that path.
Don't: Play "Sometimes a game needs to end" cards
When I hear someone say this, it is just BS code for, "I know this card isn't fun, and is overpowered, but I need an excuse to play with it anyway." And that is all it is: an excuse. I 100% guarantee that if you are about to put them out of a game, they will play that card or card combo. If a game is dragging on too long and everyone just wants it to end, then agree to end the game and start a new one. You don't have to wait until someone draws a card that is just stupid and boring so they can announce that they win because of some uninventive play made under the guise of "Sometimes a game needs to end." This isn't a tournament. And if not everyone is willing to end it, you can just concede. Start a new game with others or mess with your phone until they finally give up.
Do: Play your style
If your style is more "In your face" or just hanging back waiting for the right moment, go with it. Maybe your style is political or apolitical. Sounds good. As long as your style doesn't make for a miserable game for the people you are playing with, go for it.
Don't: Complain about everyone picking on you
Either you were the strongest player, had the strongest board state, were obnoxious to play with, or chose a play style that encouraged everyone to pick on you. Either they are complimenting you by attacking you, or you should be taking a look at how you play the game. Players don't consistently attack one person without a reason. When players attack me it is usually because I did something that suggested my deck was the most threatening.
If you are being picked on, you have likely earned it in some way. It is up to you to change their mind and complaining is not going to help you.