1. A single "combo" is enough for a competitive deck
This is a hold over from the casual play. We want that awesome combo to happen! Despite countless reasons for the combo being awesome, there are far more reasons why it is not. But the fact is that no successful decks are built around any single combo because it's too fragile, too easy to disrupt and generally too unreliable.
As much as my friend Joe wanted his Leveler + Blasting Station + Endless Whispers deck to work, however there are endless reasons this deck never went competitive. Competitive and tournament-winning decks may have a combo within them, but the deck is also designed to be able to abandon that plan and work another way or they've got numerous ways to recover from the blow, whatever it may be.
An argument can be made for control decks designed to protect the combo, but in large these decks play "not to lose" until the combo is ready to go off rather than playing to win from the get-go. Playing "not to lose" is difficult because you're putting yourself on the defensive by choice and, if your defenses fail, you are not only at a disadvantage positionally but also temporally. This is my argument against the combo/control decks.
2. It's okay for the deck to be fat
Maybe, with plenty of playtesting for tuning, a fat deck could win. But when people bring the 71-card monstrosities to me this is the very first tip I try to make them understand. Not counting the Battle of Wits decks, when the deck is fat its odds of losing go up drastically due to variance. The possible utility of carrying those extra cards is very unlikely to help you in the long run.
Magic is, at its core, a game of minimizing chance. That's really what the root of the game is. You build, play, and act all to minimize the chance of failure. If you willingly fatten your deck, you're watering down your mixture and then you're hobbling yourself before you've even sat down at the table.
Maybe you think you've balanced your deck at 67 cards. You're winning a fair percentage and the deck seems to be running consistent? Sure, it happens. Look at Butterbean, the professional boxer. He's not built like other athletes and yet he wins. How? He has a simple plan. Stand still, spin to face his opponent, and then slug them with a haymaker.
But what happens when his smaller, leaner, more agile opponents out-maneuver him? If they get inside his reach and force him to move? He's clumsy, off balance, and out of his element. This is what happens to your deck. It's sluggish. That one card you need to draw has more cards above it and your odds of finding it when you need it are lower. This is the core of the minimum deck size. You want it as light and agile as possible.
3. Losing creatures without gaining anything in return is okay
Along with the minimizing of chance, the second key to Magic is understanding parity and attrition. These are things Lee and I discuss frequently on the show. When you begin losing creatures with nothing to show for it, you're driving a car that's leaking gas: you might still finish the race but your job just got a lot harder.
This is difficult for everyone, not just new players. The difference is that more experienced players are better at simply not letting it happen. They make better choices, make better attacks and blocks, and time the casting of their spells better. But even the best have trouble overcoming attrition.
I like the gas analogy especially when it comes to this topic. We already use it when a deck has lost momentum; we say it has "run out of gas." So, when you're giving your creatures up you're wasting gas.
4. I always have to block
Following along with above, this is perhaps the biggest point where players lose creatures without reason. They make a bad block. Either they forget their opponent can cast Giant Growth, or they miscounted the damage, or they simply acted too quickly and didn't take their time considering the repercussions.
To avoid making bad blocks you have to learn when to block in the first place. Many new players block every attack, being afraid to take any damage. The first thing to realize is that it's okay for you to take damage. No boxer wins a fight without taking a few blows to the chin. The only point of damage that matters is the last one.
Blocking opens your creatures up and leaves them vulnerable. If you've got a 4/4 and they're attacking with a 1/1 you have to consider the likelihood that they're holding a trick. Also you need to consider whether they are bluffing you. It's better to take a few cheap shots and not risk losing your creatures until you have a better gauge of what is in the opponent's hand or if they are tapped out, etc.
5. Winning tournaments is easy and I should win immediately
The learning curve in Magic is sometimes enough to cause people to quit. They get overwhelmed by the many cards, the rules, the complexity, the STUFF of the game. If they make it past that, then they start trying to play competitively and people lose. There is an immense amount of ego some of us carry into the game with us and we believe that we should be able to win in a rather short amount of time.
Drafting is a perfect example of it. It's one of the most played Magic formats, beaten perhaps only by Standard, and yet it is also one of the most complex ways to play the game. You need to interpret signals from the others, pick the right cards, build the right deck, and then play it correctly. If any of those goes the wrong way then you're going to lose. And as a new player it's very hard to even realize how complex that little in-game ecosystem is.
It's a tough lesson to teach new players, but the game is hard and most of us are going to lose a lot.
Well, some of us more than others (like me) but the lessons hold true!
-- Trick