Others have been doing this and I got this topic suggested, so why not.
This is me raging / outbursting 5 the most annoying things I can think of.
(Not in any specific order)

1. Nidalee supports

JUST NO. Everytime I see this no matter if it's normals or ranked..
It makes me want to dodge. Nidalee is not a ******** support.
Go mid lane and be useless, because you can't hit spears or go AD
bruiser Nidalee top and do no damage and just jump around in
team fights and wonder why you're always the last one alive, but
don't go support.

Seriously. Nidalee supports rush a damn Tear of goddess and
buy no wards at all. You don't give any lane presence snd spear + heal
are practically useless. Lane gets automatically pushed and ADC is zoned from farm.

Second item Sightstone ? NOPE, it's a ******** Rabadon's Deathcap - Not a single
ward was placed that match. Then if bot lane diamond smurf is tower diving and you
are about to get kill on that Teemo, a damn spear appears to "killsecure" 20hp
towerdiving Teemo. After 6 - You don't have to worry about farm, because she will
do it for you in cougar form.

2. Teamselect badass

You're in champion select in ranked. Everyone communicates and you feel we're gonna
win this. Then the lastpick guy in losingstreak. "OMG DON'T PICK Udyr!
I TOTALLY WOULD'VE WON LAST 4 MATCHES, BUT NOOB Udyr WENT 1-10 and I lost.

Nobody cares how someone did at last game or what champions were picked. It's a new game
and 21/0 Vayne stats ain't helping here. First picker mains Udyr and locks it.
Last pick guy goes bat-**** insane and threats to go afk, because the guy at last game
had bad stats.

3. The kid you can't win arguement against

Let's say you win bot lane with ADC that goes 5-0. Everyones happy and then someone
spouts "You had easy lane, because Caitlyn counters Vayne so bad."

- The kid explodes with reasoning why Vayne is 1000% better than Caitlyn
in early-game and how much more skilled he is to beat "counter" lane. Suddenly he
looses attention and is 5-4 and continues to argue - In all chat.

No matter how much someone tries to explain (there is always one!) why Caitlyn
is much better than Vayne early-game, the kid just doesn't give up. This kid is
on a whole new level of stupidity and he will drag everyone down to his level and then
beat everyone with his experience. - Suddenly eveyrone is not paying attention and game lost.

4. Meeting people from your own country in-game

Me: HEY! I think there's people from my country (Let's just assume Dutch - No offense,
just a example. I will sub in English)
Me: Hey! You from Dutch also ?
Enemy-team: Yes we are - ******
Me: I just said hello...
Enemy-team: STFU ****** - LOL YOU MUST BE BRONZE

Then it's a flame war and arguing about who is better. Like wtf >.>
Things always turn hostile when you meet someone from you're country.

5. Riot favoring around countries

It's kinda obvious and understandable that Riot focuses around the main incomes.
So would I of course, but c'mon. As a player from EUW, I'm pissed that Korean server
was rewarded with free 975 RP skins for 1-2 day troubles, NA has troubles and received
2000 RP (Correct if wrong) - EUW got a damn IP boost. Now EUW servers have had loads
of problems and nothing has been done in order to relief the dissapointment of having
these server problems.