What is the one thing people do on your main that just makes you cringe?
For me it's maxing Q first instead of E

For me it's maxing Q first instead of E

Stop spamming q on Janna, especially during laning phase.
I really hope you're not maxxing q.
Starting 
Bond of stone on thresh, building tank thresh, rushing mobi boots on thresh and not fucking roaming at all, building full tank thresh with 0 cdr.........
DEVOURER ON 
on
.

Playing
and being super bm/ragey/toxic/spamming mastery emote. C'mon guys, why do you have to perpetuate the stereotype and make it harder on the rest of us.
I've gotten honors from the enemy team in a few games just for not using my mastery emote on them when I was snowballing. :/
Maxing W first.
It got poor scaling and you don't need the cdr in lane. Q and even E are better choices.
Building ad, its pointless. sion isn't very good at staying on top of his opponents, at least not in the way that lets him auto attack. Most of his power sits in his abilities. if people want to deal damage with sion grab a pile of hybrid pen. Utilize his abilities because they have high base damage. Rather then tryieng to auto atatck people to death and getting kited use your abilities to zone and kite your opponent.
And the next thing would be just stacking nothing but hp. Please get as much cdr as you can build alot of resistances and hp. You want to make your shield unbreakable by having enough armor/mr so you can absord damage with it and then detonate it every few seconds and in between you can zone and kite with the rest of your abilities. Once you cc people thats where you close in to auto with your shield up, detonate it before it breaks for some sweet aoe. Hp is nice but for a second consider your shield as free hp, having it up as much as you can means you will be more tanky, having q up as much as you can means you can zone more so enemies wont hit you, thus amking you more tanky, q will also help with kiting so they cant hit you so that means less damage taken. Having high resistances means all your hp will be stronger and all your shields will be stronger. Now when you got cdr and resistances covered thats when you can look for items which give hp.
leveling q or e is mathcup dependant often e is better for the nonstop poke, and its cost less mana. but i often see people level q regardless of matchup which sometimes makes me cringe.
obviously.
You use R as soon as they come back to lane (if they dont have a healer sup) and instantly start with an advantage. Its not going to help you in a fight, and sniping runners is pointless because if theyre running, that means you already won the fight.
is all about pushing advantage and knocking down towers.
Getting a phage when they have the gold for a Sheen.
Blue ezreal with botrk
Fizz without morellonomicon and cdr boots
Pressing Q right after using W.
I have two mains, so I'll do both ^-^
using more than one soldier to farm. You only need one to farm. The other's for an escape(Q+E combo).
maxing E first instead of Q.
Leading ganks with EQ... sorry but after 326k points i know you're supposed to at least walk up and W them
I see too many j4s just EQ in and not hit it or it gets dodged
Not building CDR
just
shivers
for me its people who dont know how to use S on 
Ganking before 6
Or ganking pre 6 with not fully stacked passive. Not knowing how to get around the nerf to bola.
Either ulting people at %65 health or taking flash/ignite with thunderlords then rushing sunfire cape. Both just make me want to smack my head on the keyboard
Building
if going lane 
Not building
or not using CDR items/runes 
For other champions that are not necessarily my main but I like playing them a lot:
Using Thunderlord's Decree instead of Deathfire Touch
and 
Building lots of movement speed items just because "hey, he has a passive" 
Not building
as a first item 
Rushing
into
with no intention of ever split pushing. Those are the items you build if you don't intend to group until late game. Yes, your dueling power and mobility with those 2 items is very good but your teamfight damage is absolutely abysmal and you're far better off going
if you have any sort of Wombo team set up.
I guess maxing W first... I'm pretty laid back when it comes to Wukong. Hell, AP Wukong can sort of work if you build something like this:

building him as a tank. there is no better way to make Gnar useless in his mini form then to build him pure tank. Tank/ADC? Fine. Squishy ADC? dangerous, but could pay off. Juggernaut? fair enough. tank? you are essentially negating half of his usefulness.
Not knowing his burst combo Facepalm
unable to hit any splash knock ups.
I don't main him, but I like that champ alot. And when people do shit like maxing Q first, I open mid, afk and write an angry letter to Rito.
Losing as Aatrox and building Tank bard
"I'm pressing E but Mark of the Assassin isn't procing wtf?"