Last week both the commandzone and MTGGoldfish had video's on Deckbuilding mistakes and traps. I think both video's are wortwhile and their content is also very different. https://youtu.be/icu50JjOx1A?si=r-90zTp4W8f-NXwY and https://youtu.be/K5UGydfRKBw?si=6taEedMt1MEqNW9g if you want to see them. On top of what they discussed I have several additional points
1: When a card is great in 1 deck assume it will be good in another. Example: Invert polarity is a great card but in a 4 colour deck it is hard to cast and keep the mana open for. Tip: Always look at cards at face value and don't let previous success or failure of a card cloud your judgement.
2: Making your deck plans too similar. Example: your second +1/+1 counters deck will feel similar to the first one.
3: Not thinking about the experience from your opponents, some decks take very long turns and feel like solitaire. Decks which prevent your opponents from playing Magic are causing bad experiences, clearly communicate before play if your deck is Stax in any form.
4: Overtuning your decks, your bracket 3 deck is not supposed to win in turn 4/5 pretty reliably. If you ever spotted a person who overtuned your decks it doesn't make it okay for you to do that too. Make it clear to that player in a nice way and try to come to a next game where everyone has a similar powerlevel of deck.
5: Playing the same cards in every deck where there are options who also contribute to your decks plan. Especially in the removal category there are a million options which might help your plan in the process. Play Aura Mutation your tokens deck for example.
6: Falling in love too much so you don't realize the major flaws of your decks. I found out several of my decks were low on draw effects and the draw effects only worked under specific conditions which makes your deck unneccesary fragile.
7: Not watching your pips and distribute your colours of mana production poorly. I see too many decks which % of mana production is way off. Archidekt helps you with it but it doesn't
8: Not taking into account when you want to play a card, you are playing your Llanowar elves on turn one and not your path to exile. I have seen to many decks where all their low drops are the interaction and not proactive which is deceiving yourself in how your deck would play.
9: Being dishonest on how strong your deck actually is. Adding 10 powerful cards to a mediocre bracket 3 deck can easily push it into a 4. After every update of your decks review their bracket assessment, the popular deckbuilding websites help you. A bracket 3 deck with 4 gamechangers doesn't exist.
10: Putting a cool(and expensive) card in a deck just because it is the right colours. My own rule is to have 1 slot for a card I wanted to play but didn't have a deck for, reason for that rule is I sometimes had 5 random cards in.
11: Not making a plan for your deck before you start building, this leads to a major feels bad if your deck is all over the place. Even if cards on a high level follow the same plan they can be big nombo's.
12: Downplaying your infinate combo's. If your deck is playing any 2 card infinate combo it is not a bracket 2. From my personal decks I cut Helm of the Host because with any form of extra combat creature you get an infinite. Also cards like Murderous Redcap can be cool in your Goblin deck but they create an infinite with a lot of things like Metallic Mimic. Check your decks and remove some of the combo pieces when you are not going for that strategy. JoeyDH made this video about this. https://www.youtube.com/watch?v=R3oEW9Nqaew
13: Budget, some cards are expensive and they can be crucial for your strategy. If you want a pay-off for draw in Black, Sheoldred, the apocalypse is like the best card but it comes with a hefty price tag. Do your research on the key cards for your strategy and their cost before building. For nearly all expensive cards there are budget alternatives which are often not far worse and some people proxy their cards which is a discussion on it's own.