Honestly, it's not even the Windwall that's an issue on Yasuo, it's literally just his shield. Using a range advantage to poke him doesn't do much since his shield can eat two autos or an ability at level 1 and he can have it back up in 5-8 seconds by just pacing.
It makes a window to trade him very difficult to find since, if you do commit to a trade, you automatically start 2 autos or 1 ability down in damage when he gets on top of you and when he does, he deals more DPS than any champion in the game pre-6 simply due to Q spam. So if he dives on you while he's at 30% HP, if he has his shield, he can probably kill you from full before you can even get down his last 200 HP.
I played Yasuo for about 10 games and won 8 of them hard carrying. I don't even play melee champs hardly and I'm only Gold. I really couldn't tell you his weakness' either. If you play like you have a brain stem and don't overcommit to every fight, you can kinda win by just stat-checking the enemy in 1v1's. The enemy team can't actually do anything and the only way you can lose is if you actively make a mistake. He pretty much prints LP and players who main him are probably a whole tier above where they should realistically be if they mained any other champion. Yasuo mains go ahead and flame me. Lol.
Now, with that said, there are things you can do that can force a Yasuo player to make mistakes or at least keep him from affecting the game outside of your lane.
Most Yasuo players sub-Diamond are extremely aggressive, which is not how he should be played all of the time. If you're playing a mage, you need to just back off of the wave. Do not walk up and let him dash through minions to you or you'll lose half of your HP putting you in an easy kill range for him on his next trade. If this happens, back off the wave entirely. Sit under tower. 100% chance if you're at 50% HP with Yasuo, he'll wait for his shield and as soon as it's up and you step into range he'll flash > dash > auto > Q > ignite and you die every time unless you're running barrier, which you won't be because you should probably be running exhaust for him.
Rule #1 about a Yasuo in lane as a mage is to just not fight him. There is no situation you actively win a 1v1 without him misplaying extremely badly. Your goal needs to be vision control and keeping the wave off of your tower. Use your abilities to clear minion waves and get deep vision in the river and enemy jungle. Your job is to contain the Yasuo. Not feed him any kills by playing like an absolute bitch unless your jungle ganks.
Don't try to deny him CS, he'll take it anyways and then punish you for trying. Don't try to trade with him, he'll punish you for trying. Roaming is difficult against a Yasuo because he clears waves very fast, so either be smart about it or he'll punish you for trying.
Get as much CS as you can without putting yourself at risk for a trade or all-in. Ping like a fucking lunatic if he leaves lane and always keep pink wards stocked in your inventory to keep in river. If you're playing super safe, then Yasuo is going to want to just shove in the wave and roam. You can only keep the wave pushed with abilities for so long before you run out of mana. Yasuo might not be able to beat you in shoving waves, but he will definitely outlast you and when he does, you need to have vision down to see where he tries to roam to.
If he can't pick up kills in his own lane then he's going to be looking for kills in other lanes. Your job is to deny him getting fed. Yasuo relies on getting fed in the laning phase to have a reasonable impact on the game.
Now, you are going to be about as useful as Yasuo is to the game if you successfully deny him kills and roams. If you get fed, you're lucky, but just expect that you won't. Fortunately for you, though, mages have utility where as Yasuo is all damage. It is not unreasonable to build an ancient coin or something like that on your first back. Just something so you have some more gold transitioning into the mid game. You're not going to be very aggressive anyways so building damage is kind of just yet.
So yeah, the best counter to Yasuo is to avoid him and ping your team so they know to avoid him as well. It's pretty sad that the only counterplay to him is to not get near him, but what can you do. It's not like Riot's actually going to rework him.
So good luck in your games.