HORRIBLY unbalanced heroes, you will see even though there is a plethora of heroes, you will always see 3-5 heroes, because of their OP-ness.
There are also invincibility frames, some hero's auto attack combo gives invincibility frames that last longer than another hero's skill invincibility frames.
Paying players have a HUGE advantage over free players, (Ex. only way to get perm heroes is to pay, you can also buy fully upgraded gear with real money, there are also a group of heroes you can only get by paying, which have some of the most broken skills)
Combat system is based on spamming/stun locking/knocking people off the field.
Even though it's a team game everyone gets mad if you aren't 1v1ing even if the teams are unbalanced, Ex. 5v3
Lag can help you lag-out of attacks, or be your instant death as the animations of skill won't load until after they hit you.
The concept of teamwork or doing anything like kiting will have three 12yr olds yelling at you and trying to get you kicked from the match.
One of the most toxic communities I've ever seen, I'm guess most people that play it are 12-16
It's VERY hard to report people as there are combat conditions (like who is winning and who has the most points) that will prevent you from reporting even the most toxic of players.
Not everything is translated properly, and plenty of text issues (like text running off the screen or skill names overlapping)
Camera is slow to follow hero, so if you run into another player's field of vision they can see you before you see them.
Despite being 10 game modes (prisoner, crown control, ect) you will only ever find a group for 3-4 of them