As a GK player myself I have to say I really miss my old daemonhunters dex. It made it really seem like the numbers were against me when I played them, though they seemed to have a problem keeping up due to wargear.
Here are my personal thoughs of what they could do and not hurt them too bad but bring them back in line...
Nades
I wouldn't mind if they got rid of psychotroke nades completely, rad nades aren't as bad in my opinion since the old Grey Knights had str 6 weapons, those combined with hammerhand brings that back to what it was only with guys that have weapon skill 4 instead of 5 so it's a little less of an affect. On the flip side they're all force weapons.
Nemesis Force Weapons
To address that I would be fine to have the strike squad guys have regular weapons and have the justicars have power weapons like before.
Halberds could in my opinion be dropped down to only adding +1 initiative instead for the +2 and keep the same cost.
Units
Let all terminators be the weapon skill 5 and keep force weapons just increase cost by 5 points each. Since they're supposed to be the best that GK have to offer. Since they don't have storm shields as an option I don't think that's unreasonable, going along with the idea that halberds only give
Move purifiers cost up to 35 points each if and increase their weapons skill to 5. Also keep it to 1 special weapon per 5 guys, they're not a heavy weapons team.
Increase cost of psybolt ammo for Dreadnaughts to 20 points.