This would be considered a guide I suppose. I've seen a lot of the following misconceptions said or done by one of the many 4th gen competitive posters recently so maybe this will help.
Common Misconception 1: Team Building
The first common misconception on the list is that every team you build has to look like this.
1 Physical Sweeper
1 Special Sweeper
1 Physical Wall/Sponge
1 Special Wall/Sponge
This is kind of old and outdated but a lot of people still go by it. To start with, many Pokémon are able to do multiple things at once. For example, Snorlax is a great Physical Sweeper, but it can also wall a majority of the special threats in the OU metagame. The second thing wrong with this way of thinking is that it implies having a dedicated wall is automatically going to shut down all special threats. This is obviously wrong as even Blissey can't wall CM Wish Jirachi, or SubSplit Gengar. Skarmory can't even wall things like LO Gyarados and SD Lucario. Finally, not every team needs to have a special and physical sweeper to be effective. Hell, with Blissey's dominance, a majority of OU teams lack the former. Always look at Pokémon individually when making a team; The above format is wrong.
Common Misconception 2: Annoyers
People, annoyers do not exist. Annoyer is a stupid name for things such as Confuse Ray Umbreon or Sub T-wave Jirachi. They are not there to "annoy" the opponent, they're there because they can actually beat something with the odds being slanted in their favor thanks to Confuse Ray/T-wave's effect. The term "annoyer" has got to be scrapped. An annoyer would be some stupid **** that just tries to get your opponent to ragequit or something.
Common Misconception 3: a 2X Super-effective Hidden Power compared to a STAB 95 base power attack
I often times see people saying they have Hidden Power Electric on Vaporeon to hit Skarmory or something. What they don't realize is that with STAB factored in, that neutral Surf will actually be doing more damage than the Super-effective Hidden Power.
Surf (95) + Stab(47.5) = 142.5
HP Electric (70) + Super-effective = 140
The numbers don't lie. With this in mind, it is generally only recommended to use Hidden Power to capitalize on things that are 4X weak to the type. This has a few exceptions though. For example, on Vaporeon, other Water type's will resist the rest of the moveset, so by adding HP Electric you have insurance against other Water's as it nearly double's the NVE Surf's damage output.
Common Misconception 4: EVs
Never put 255 EVs in a stat. You don't get any benefit out of them so you might as well use 252 and get your extra point. 252/252/4 is much better than 255/255. Every point can matter in Pokémon.
252 /128 /128 spreads are hardly ever the right spread. Even when you want to split the defenses it usually better to just go for a jump point in one of the defenses, thus maximizing the spread's potential. However, some people overate the value of these jump points. A good example is Swampert. It may seem optimal to only put 216 EVs in Defense and the rest in Special Defense, but really, Swampert needs all the Defense it can get. Swampert is constantly taking boosted attacks from Tyranitar and Metagross, some of OU's strongest attackers, and every Defense point matters in these situations.
Another thing I've seen involves Pokémon being used in conjunction with Abomasnow/Hippowdon/Tyranitar. On teams with said Pokémon, oftentimes people will go for a Leftovers number when the Pokémon isn't immune to the weather. The problem with this is that you're gaining the same amount of residual damage. In other words, it causes you to lose more from Sandstorm/Hail as well.
Common misconception 5: Checks and Counters
So many noobs use this for their reasoning that Salamence shouldn't be uber. Well noobs, Weavile is not a counter. A counter is a Pokémon that can SWITCH IN to another Pokémon with little to no risk to its own well-being, and also immediately threaten the opponent. Weavile cannot take anything from Salamence. Not all Salamence are Dragon Dancers. 50% are mixed. So when you bring in your Weavile, don't ***** you lost your "counter" when it is KOd by Draco Meteor. So, Weavile is not a Salamence counter, it is merely a "check".
On a side note, a Pokémon's performance in Ubers does not affect its tiering.
Common Misconception 6: Good on Paper = Good in Practice
It's appalling how many RMT threads on Serebii feature two certain Pokémon. These Pokémon seem quite good on paper, but in practice they are terrible. I'll start with Electivire. Electivire is the epitome of "It's super-effective" not amounting to a KO. Sure his attacks get good coverage, but if you go physical, you have to deal with how weak 75 Base Power is. If you go special you have to deal with Electivire's below average Special Attack stat. Electivire is also kind of slow. Anything slower than Electivire can usually take its attacks do to the aforementioned reasons, and anything faster can usually OHKO it.
Ninjask, also sucks. No competent player will ever let a Ninjask pass boosts. 1 of 3 attacks will be on nearly every team. They are Roar, Taunt, and Whirlwind. All of these moves equal an instant shutdown for Ninjask. If usage stats were weighted Ninjask would be UU where it belongs.
Dusknoir is another good example of paper differentiating from play. On paper, it looks like Dusknoir would be giving Rotom-A some stiff competition for the number one spin-blocker in OU. It's higher defenses look good at first glance. But if you look closer you can see why you wouldn't want to use it on a team requiring spin-blocking. The biggest issue is lack of Levitate. This hurts against opposing Stall and the like. Coming in on a predicted Rapid Spin isn't doing much if you can't do it repeatedly. Taking damage from both Spikes and Toxic Spikes means Dusknoir will only get a few chances to come in, and even less if the opponent predicts right. Rotom-A has an immunity to the grounded Entry Hazards, and coveted resistances to Steel and Electric. Being able to come in on random Earthquake's is a god send as Earthquake is the most common attack in the game. When using Dusknoir you're gonna wanna focus on making sure it isn't an inferior Rotom-A. Focus on it's physical attacking prowess. Dusknoir is one of only 2 Ghosts that can utilize Physical attacks decently. A SubPunch or Choice Band set could be viable given the right support. Access to the elemental punches and Earthquake allows to function as a utility counter of sorts, something that Rotom-A can't do.
Common Misconception 7: Creativity is key
This is probably one of the biggest problems this forum has. People think that if you throw some random UU Pokémon on to a team, that it's creative and/or viable. Originality for originality's sake is bad. Being original simply for the cool factor is a bad approach to competitive Pokémon. Now don't get me wrong, there's nothing wrong with being creative if it works. However, many times people will slap a few UUs on to their team and say they're creative battlers. But most of the time, they're just using outclassed stuff such as Cloyster, or Blaziken. In the end that's not original at all as they run almost the exact same set as their OU counterparts. If you're going to use lesser used Pokémon, make sure they aren't being outclassed by another. In the end, simply being original is not going to win you battles.
Remember, standards are standard for a reason. CHOICE SPECS WEAVILE KILLING SCIZOR WITH HP FIRE!! might be a "surprise" to your opponent, but it comes at the cost of completely nerfing your Pokémon. Obviously not all examples are this dumb, but even in cases of a random move tweak to a more gimmicky option that can surprise a common counter, make sure your team can actually capitalize on this. Your Tyranitar might think it's cool when it gravely weakens a Swampert with HP Grass, but your Shaymin would be facepalming if its arms were long enough, as it knows it's not any closer to sweeping. *Credit to bonslet for this very informative paragraph
Well, I hope this "guide" will help some newer players, and maybe even some veterans. I actually kind of enjoyed writing it. If anyone else can think of more common misconceptions I'll gladly edit them in, just post letting me know.