These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Base Breaker: The introduction of the Technician, which sharply divided most of the community into two camps, one that thinks the Technician brings intricacy into the strategy of team composition and fighting; and the other which believes Techs dumb down the course of play by making HP recovery (something which had to be managed very carefully) simple, and forgiving of players who make mistakes by being reckless. The biggest complaint is that it is the only class which is essentially viewed as able to change the balance of a match just by being present, thus the only way to fight a team with a Tech is a team with a Tech.
The EMP, at the launch of open beta, was really broken. It shut down a mechs weapons for 7.5 seconds, traveled very fast, and had a blast radius of a small planet that was much bigger than the animation of it. Getting hit with this thing was a death sentence without countermeasures. Later nerfed to reduce its speed, size, and time stunned to 5 seconds max, but some still consider it overpowered.
On the weapon side of things, the EOC Repeater. Deals a ton of damage while charged, generates very little heat, and has a very high fire rate. Mad worse by being attached to other game breakers.
A class mechs: Small, fast, and not packing much armor, but way better weapons and abilities than B or C class mechs. Of note:
The Infiltrator packs the aforementioned EOC, has a disproportionate amount of health for an A class, and is very fast with the ability to cloak.
The Scout. Has a the flak cannon and the tow rocket, and despite the low amount of armor, has one of the best dodges in the game.
None, however, is been more hated than the Technician. Despite being a healer, it is still a potent attacker, can heal itself while healing others, and has the Amplification ability, which heals its target extremely fast, basically making the player being healed by them them immortal for a short period.
Irony: Raider is a repurposed merchant escort mech.