sort of a TL;DR at bottom and this is coming from an avid player of Jax but not pro or anything like that.
I agree Jax is a little on the strong side and could use some tweaks and adjustments here and there BUT...
First, It's not just him, it's masteries, items AND player incompetence that is making him so strong.
Now what do i mean by "player incompetence"? I'm not saying you're stupid for losing against Jax. What it means is not running away from Jax when he E's, grouping up and getting AOE stunned by Jax, NOT using AP against him, AP hurts Jax like a motherfucker and before you say "but jax can just build magic resist", well while he's building magic resist he's: not building
or
the worst offenders as you'll read in point 3 and he's not building armor, now Jax doesn't need armor as much as other champions do because of his E but it's not like he doesn't need armor at all and in point 2 you'll see why.
Second, Jax is a lot more vulnerable while is E is on CD, of course he isn't completely defenseless without it but people underestimate how vulnerable he actually is without his E.
Third, Items and masteries also make Jax a lot stronger
+
being the worst offenders and Conqueror as well.
Now there is prob more i could say but i just wanted to say something for now and let me say this again i DON'T think Jax is weak he IS strong right now but it's not just him it's: Items, Masteries, and people not playing properly around Jax AKA "player incompetence" as i like to call it.