Can't forget the old "love interest is the childhood friend" cliche.

I don't think it's limited to just shounen either.

I'm surprised no one's mentioned it yet...

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When a character goes through so much just to reach that next level (Goku going Super Saiyan, Ichigo achieving Bankai, etc.), and once they achieve it, they can switch from normal to next level and back to normal as easily as you flipping a switch to turn the light on and off.

It's always so amazing the first time they do it (and even the second and third), but after wards they do it so much that it loses its' luster, and you grow bored with it thinking to yourself, "Am I supposed to be excited about this?" or "Why was achieving that next level so hard again?"

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Haha the worst is
The wuss main character than becomes fricken godly later on
Gohan >.>
Sena <.<

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Well, Gohan's understandable. He was thrust into battle when he was 3 years old.

But Sena...he has no excuse.

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hey man~ sometimes we do need that cliche
like all the kamen raider always have to do somekind of weird dance to transform

the worst shounen cliche :
in harem manga, many beautiful and hot girls love a single loser yet ugly guy

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perhaps not the worst cliche, but the one that pisses me off the most is where there is an explanation of powers or whatever in the first ten pages of the first chapter. i dunna. hate it, but kinda necessary

An idiot male lead. I mean that as in someone who doesn't realize the situation/can't decide between girls/learns slowly/doesn't follow long conversations well/loses his resolve at some point.
And yes, I said male lead, the female lead shounen mangas are usually better

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Lalala~

The worst cliche?The "I Won't let you hurt my friends" or "My Friends give me power!" bullshit

u yeah~ absolutely...

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It's definitely idiot male lead. I'm thinking "Unless the bloke has got 10 mil in the bank those women around him must be androids".

When a harem owner just DOESN'T do anything with one of the many many chicks that like him

or no, this one:
A female childhood friend who is in love with the main character and beats him up whenever other girls are touching him. And the fact that almost all of those childhood friends are flat chested.

i agree with many cliches above...
i hate harem, specially the ones with beauty dumb girls, and protagonist that never die, and when they are about to do it, they magicaly get stronger... it doesn't make any sense , or when the mangaka tries to make the story incredibly long, introducing things not related with the main plot, like useless characters or stuff like that...
still, i keep reading it

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funny because you end up having a harem main that does do things with the many chicks and you end up with an ending like school days, or tenchi muyo gxp.

Maybe shounen main characters don't want to end up like Makoto, and that's why they don't mess around with girls.
School Days really is one fucked up manga