No one mentioned the disruptive nanites. Cheapest gamechanger in the game. You only need one turret to shoot one shot at a capitol ship/titan to disable its passive regeneration for 5 minutes.
Suffice to say, Rebellion added a ton of incredibly powerful abilities, especially AoE. The Kostura in stock sins destroyed fleets, and the first time I saw a hostile starbase was a sad day. It also startled me when I first got a supply pact and got an extra thousand fleet supply. However, I still feel that the titan and the corvette are the most overpowering changes to the game; second online match I couldn't manage to defeat a constant swarm of Vasari corvettes, which constantly fled pursuing ships and raided random systems, were easily replaced, and could overwhelm a damaged capitol ship or titan. And titans are essential in Sins; winning without one requires more resources or just more skill than your opponent; otherwise you are seriously outmatched.
I'd be content if corvettes, disruptive nanites, and pirates were nerfed. Even if you beat late game pirate fleets, the opponent can win by crashing your fps with multiple fleets containing hundreds of pirate ships (and the worst abusers? BOTS). I can't play with active pirates anymore without seriously planning against them, disruptive nanites are just insane for level 2, and corvettes are too universally useful. Otherwise, the overpowering abilities each faction has at least make the game interesting.