Unbalanced classes and lots of grinding
It takes hours upon hours to lvl up, though like other MMOs, the next level is only ever a handful of matches away. It's just a shame that the robotics class so utterly dominates in PvP battlefields. With their abilities to deploy gun and rocket turrets as well as powerful forcefields, both they and their turrets are individually able to outdamage a player with a medic healing him over time.
Considering most of the maps are control point based, it can frequently come down to which team's robotics got there first. Before I uninstalled it, I found only joining match queues with < 3 robotics helped somewhat.
Recons have no stealth or melee +damage bonuses or specific anti-turret abilities, a casualty from the TF2 "borrowing" process perhaps. Medics are quite fun to play and are capable of the same level of damage output as recons too. Assaults have a very overpowered "invincibility" power, useful for holding ground for a time.
A few balance changes would help the game significantly, but between the very unbalanced classes, the grind, the PvE AI and poor rewards as well as a useless crafting system and horrid auction house (just try finding something), the fun and clever parts of the game are quickly forgotten to be replaced with sighs, bewilderment and frustration.