Counter-abilities are definitely something to consider. It’s important to have passive/general/tactical counters to abilities, as well as direct ones.
What I mean is that if turrets can have a high skill ceiling in placement, spacing, upkeep, etc… that makes the engineer a class (or in this case, a character) worth choosing. The correct balance is that a character like T-Rat placing deoplyables haphazardly is ineffective, a T-Rat player that can place turrets properly and has good gun-play can be a strong opponent at a choke-point that takes maneuvering and a teammate or two to clear out, and a really effective turret manager can shut down an enemy offense, and requires a direct counter like EMP grenades, or a cloaking ability. At that point, it comes down to a firefight between teams, supporting the defensive specialist, and other needing a solid covert-ops.
Another benefit is that it takes a moderate-skilled covie to disable an excellent engineer, but it would take a very talented covert-ops player to cause massive disruptions to the enemy offense.
At the moment, turrets are kind of a joke in a skill-level sense. Really all you can do is pick a slightly better spot, and defend it a little better. Mines need this type of overhaul too.
This was the meta-game balance that was achieved in Quake Wars. I feel like Dirty Bomb is getting to a point so far superior in performance and netcode (UE3 does have its own issues like texture streaming, mouse input, rim lighting, wacky netcode… but the issues of IdTech4 were far more hard-line limiting) that it can bring the modern spectator-friendly MOBA presentation, the gunplay of W:ET, and the amazingly deep meta-game of Quake Wars together.