For tournaments, these mistakes mostly apply to Limited, but they are frequently true in one-on-one casual play as well.
Not Attacking Enough: Mike Turian, a Magic developer, once told me that this was the number one mistake newer players make.
Here, I think the methodology mistakenly being applied is "better safe than sorry"; once the board gets a little hairy, newer players tend to avoid making big, calculated attacks and rely entirely on evasion creatures. While this approach is correct in many situations, other times it is crucial to know when you can push, and with what, in order to have a positive exchange on a clogged board. Even if you're certain to lose a creature or two, the corresponding devastation might be worth it.
Exercise these skills. Find yourself in a situation where you're not sure if you can attack or not? Write it down and figure it out later. Another way to practice is shuffle up a bunch of creatures and put them on each side of a board. What can you attack with? What if you change the life totals?
Turian's advice? Attack with everything every turn; it would be less of a mistake, and certainly a more active learning experience!
Chump Blocking Too Much: We've all seen it before; a 4/4 is attacking the player on 18 life, and he blocks it with a 1/1 (or even 2/2). What's wrong with this play?
I think this mistake comes from a general misevaluation of commodities. Yes, the game technically ends when your life total is reduced to 0. But protecting your life total is much more complicated than simply staying at a high life. A stable board will go much further toward sustaining your life total. There are a few reasons to avoid throwing away smaller creatures early in the game: First, they could later combine with another creature to gang-block something worthwhile. Second, they might be able to get in a few points of damage before chump-blocking. Third, you can usually get the same "deal" later, and an even better one might also come along (e.g., a 6/6 attacking). Chump-blocking too soon can lead down a slippery slope in which a minor disadvantage compounds into a major one.
There are exceptions: Racing against evasion creatures, you might want to take any opportunity to stop damage that you can prevent. You also might want to chump block sooner than usual if you think your small creatures are at risk (from, say, Prodigal Pyromancer or Pyroclasm). Also, 0/1 creatures (like Kobold tokens, or Birds of Paradise that are no longer useful for mana) are a lot more expendable than 1/1s...
Walking into Traps: New players are notoriously easy to set up. Perhaps this is because they mistakenly presume the game is simpler than it is (and rather than being set up, their opponents might just be making random plays), or just because they are unfamiliar with the traps, and wouldn't even know how to play around them if they wanted to.
In any case, in general, you should give your opponent credit for the cards they are representing. Your decision should center less on "does he have it or doesn't he have it", and more on, "he probably has it; should I block, or not?" Bluffing happens in Magic, but it is much rarer than someone just having the straight goods.
This is because it is rare in Magic to be getting the right reward (it's usually a few points of damage) for your high risk (it's usually a valuable creature), especially considering the fact that even if your opponent gives you credit for the card you're representing, they might still block because of certain cards in their hand.
Your opponent has a full hand but isn't playing creatures.
Potential trap: they're setting up a mass-removal spell.
Counter: put out enough creatures to put pressure on them so that you'll still win if they don't have it, but hold back everything you can.
Your opponent attacks with a small creature into your bigger one.
Potential trap: they have a pump spell.
Counter: Determine if it could be a bluff. If it probably isn't, decide whether it's more profitable for you to block now and force the trick, or to take the damage and address the creature and trick later.
Your opponent passes the turn with a full hand and mana up, against your attackers.
Potential trap: They have a devastating combat trick they've saved mana to use.
Counter: Especially if you can anticipate what the exact card is (Chastise), you can decide to attack accordingly, or even not at all! Adding something to the board now will make your opponent's situation worse, and they might still have to leave up mana next turn.
Just because a play is big, clean, cool, unlikely, or generally profitable doesn't necessarily make it the best option. Evaluate potential risks as usual, decide whether the timing is right, and make sure the boring plays aren't just plain better.
Over-Prioritizing Expensive Spells: Last week we looked at some of the more subtle value in cheap spells. They can be played with a lot of flexibility, filling holes in your mana curve as necessary. Expensive spells have a corresponding handicap—they are difficult to play efficiently. They clunk up your hand. There are also of course the more obvious risks, such as never even drawing the appropriate mana in the first place, losing before you can play the card, or having it destroyed by a much cheaper card.
Many expensive spells are worth the cost and risk. But make sure you do realize the disadvantages and don't go overboard.
To a degree, this also means that newer players are more easily seduced by the big creature color—green. Big creatures aren't as good as they probably first seem.
Being Transparent: Newer players sometimes wear their hearts on their sleeve. Their opponent gets a quick draw, and they slump in their chair, with no hint of being disingenuous. Now their opponent can commit even more creatures to the table without having to worry about mass removal. Your facial expressions and mannerisms can give away a lot of relevant information. It is important to recognize this, and to reel in emotional and physical reactions to the board state.
Even if this game's a lost cause, keeping your guard up has value. Otherwise, next time, if you do have an answer, your opponent will get suspicious when you don't seem defeated.
Remember, always to adjust to the specific.
Pursuing Unlikely Game Plans: When devising a game plan, it's important to choose one that is both plausible and legitimate (i.e., will actually win the game if enacted).
Sometimes, I see players either build their deck, or center their in-game strategy, around a plan that is just plain unlikely. If it's your only option, sure... but otherwise, try to figure out something more workable.
Other times, I see players adopt a game plan that is neat and efficient but not actually capable of finishing the opponent off. They'll get their opponent low, and then lose.
Example: Your opponent has Akroma's Memorial in play and attacks you with some creatures. You use your last bounce spell to return the memorial and kill off a few creatures. He plays the Memorial next turn and wins anyway. Your only shot was to wait until you drew one of your Logic Knots, and then bounce the Memorial.
Concentrate on finding your best way to win. Unconventional is fine. Unlikely and ineffective is not!
Including Too Many Cards Quickly Made Obsolete: While it is true that cheap cards have increased flexibility, it is still important to make sure that their effect is suitably substantial. The problem with junky cards, basically, is that they can become nearly worthless against the most basic defenses.
A good rule of thumb is to only include cheap cards that either have the regular potential to trade for a more expensive card (Savannah Lions), facilitate more expensive cards (Llanowar Elves) or replace themselves (Peek). Avoid cards with marginal application; more often then not, they're as good as a mulligan.
There is a fine line between small, cheap and efficient, and irrelevant.
Using Shaky Mana Bases: I sometimes see newer players playing even, three-color splits in Limited. Having a stable mana base—being able to play your spells—will give you a lot of wins that may not be immediately obvious. It's important to prioritize a good mana base.
That's it for this week. Join me next time when we... oh, well, you'll see then, won't you?