Infiltrators aren't good because they're necessarily overpowered, they're good because while they're cloaked allies are taking all the fire, as a result can't return any, leaving everything for the Infiltrator. They're handy for those 4 data console objectives and reviving players, but apart from that infiltrators can (though certainly not always) actually be detrimental to the overall health of the team. The player they may be constantly reviving might be dying because they're taking all the enemy fire while the Infiltrator just sits in cloak and fires the odd power-shot.
This can't really be solved since it's a big part of Tactical Cloak. What there needs to be instead are better ways for other classes to benefit the team rather than the self, while still increasing player score.
Anyway, I'll just run the list of general buffs I think there needs to be.
Biotic Bubble should block instant kills.
Blade Armour should return 100% (then more from the upgrade) of melee damage. It should also effect instant kills, effectively bringing down whatever brought the user down simultaniously.
If you're in the middle of a heavy melee, even if you get synced to the enemy unit the attack should continue and strike the target, potentially stunning them and freeing you from the instant kill.
[I don't like instant kills]
Pull should stick to targets, slowing shielded units down and initiating biotic combos on all enemies.
Singularity should detonate if a biotic power passes, or hits, in its area of influence, not if something in its hold is hit. So, if a shielded Marauder walks through a Singularity, if a warp is fired at it, the singularity still detonates. It can also, therefore, be detonated even if nothing is nearby.
As an aside, I'd really like to see some "classic" ME1 Vanguards. Forget this Biotic Charge stuff, let's see a combination of Biotic and Soldier abilities, like it's supposed to be.