Things in maplestory that rustle your jimmies.

Trading someone and having them cancel when you ask for an offer.
FM spambots
Being denied entry to a party because "waiting 4 bish" or being ignored entirely.
Hunting for something for two hours and having some random kill a mob and get it.
Oh, and people who act like jerks to new players.

"brb bathroom"
*afk for 30 minutes*
*kicks from party*
"wtp! I just got back!"

scammers.

people who camp your FM spot.

people who DC hack FM just for pomegranates and giggles.

people asking "u ofer" no matter whether you're selling or buying.

extreme lowballers.

people who make an offer on an item, ask you to hold it for two days, then tell you they already bought another item when you contact them again.

people who spam a million lines for no reason anywhere that isn't FM Entrance.

Spambots inside FM rooms.

People who hold empress for the full hour because "they might have a friend who wants to get in".

people who defame for no reason.

the amount of nx it costs just to get a useful potential line on additional potential.

how there's more shitty lines in legendary potential than there are in unique potential.

I can do this all day.

obtaining NX.
Late people
spam in BL/Guild/alliance chat

caps lock in either of these.

The people. Does it matter where they are? Everyone's pretty pomegranate.

Reliance on cash.
Beggars.
People who ask really dumb questions like "is it u can advance at lvl 120?"
Expedition crashers.
Spammers.
"Offer me pls" "Srry too low"
"CC pls", especially when done at PQ areas and it's obvious their party can't even go in or they'd be in. wtp?
Racists.
Overly expensive items.
Storyline...or the lack of it for non-Hero classes.

I could go on alllllllllll day.

How basically everything I do has a chance to DC me.

Having a crappy alliance or having a crappy guild in your alliance, OR that one idiot in your guild that no one wants to kick even though he drives everyone nuts.
Towers in Aswan ignoring blinking frames, allowing the same attack to occasionally hit you twice.
Hilariously bad scrolling luck/RNG.

Many other things that have been said so far in this thread. D:

When a person sends a trade/party invite again after you decline.

Not being able to share cash shop items between different types of character on the same account.

a few of the above reminded me of my biggest gripe:

maplestory management.

Trying to cube items when you and Nexon both are very aware that most of the time you are just throwing money down the drain for little to no reward.

All these lines of potential.
Smega whores.
Boss hoggers.
D/C hackers.

Hackers, all of them, in any form.

potential, enhancements, legend classes, ksers,

When people say "cc" when they mean "channel".
Is it really that bad to just put "ch"?

When someone comes into your channel and asks you to CC

When someone begs to come to your boss run then complains that your killing it too slowly

When someone asks for GPA leech and complains when they FJ into the boss and blames you for it.

People that take up more than 1 spot in the FM, and then to top it off, don't even have anything to sell.

People that take up more than 1 spot in the FM, and then to top it off, don't even have anything to sell.

S> Spot!

Fucking mushroom vendor.

"cc pliz"