90% of all vaynes I see can get themselves fed and therefore BE overpowered. That's all I'm saying. Is she OP? I still think she is; her ability for true damage is just ludicrous. My opinion on her is like my opinion on Irelia, and her true damage ended up getting nerfed. Of course, I think 30% of the new champions that come out are just overpowered as it is, but this is my opinion. I'm tell you what I think, because I want to state my opinion, because I believe it is correct. You can disagree with it, and I respect that.
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I can't disagree with what you believe, because its what you believe, I can't contradict it. This is what I think:
In my opinion, for a champion to be overpowered, the champion must be able to output large amounts of damage even under countered conditions.
An example of a counterable champion would be Annie. Few months ago she was considered one of the strongest casters, but there exists champions such as Kassadin to shut down said champions. So is annie overpowered? no, because there are champions to counter her, she only fits her role as a mage.
Now a special case is Irelia. She was truly the FoTM, for one reason: 3 types of damage, a reduction of cc from her passive, and her tanky-ness per lvl. Her Q was the gap closer, the W was the truedamage/lifesteal and the E was the cc. She excels in her category Of course she needed a nerf, there was barely any counter to irelia, she was a pubstomper. She gets in your face. You can't really say anything about Irelia because she does her job very well, to focus the carry, her main role. Now Pre-nerf and Post-buff Irelia OP? I can agree with you there.
Now, compare Vayne to Irelia. True damage check, mobility check, 1 cc check, 1 escape check and no tanky ness. However she can be cc'ed. So what makes her excel in her category? The damage. Which got slightly touched to jump down to the same level as other carries. But, Vayne can be focused down just like Ashe and Corki, so in the big picture, she's just another carry. Can she 1v5? no, thats not her role.
What I'm trying to say is this, champions can't be considered overpowered, because they all have their roles to fit.
TL;DR: You gotta consider cs, gold, level, and team role to classify a champion as OP