I mean the guys are supposed to be major bad-a$$es.
So here are my observations:
1- TOO EXPENSIVE! I mean they are great, but they cost to much (wallet and points). So what needs to be done is reduce the points cost. I say 20 points a model, instead of 25, and only +10 for a Justicar instead of +25. The cost of their special weapons also needs to decrease. They are just way too much. The only fairly priced things are Land Raiders (if they get weapons updated) and Assassins.
2- TOO SLOW! They can not take any transports and any Deep Striking marines are not scoring. I kinda get the no transport thing, kinda, BUT the Grey Knights are supposed to be the masters of Teleportation. You read the newest Word Bearer novel. They teleported directly onto the bridge of a CSM warship and slew the entire bridge crew, including the Dark Apostle, without them having a chance to resist.
What they need is scoring deep strikers. And they need something like the Blood Angel's Descent of Angels but for teleportation.
And making it easier to take transports would be nice. If they have Land Raiders they must have rhinos at least fluff wise.
3- NO VARIETY! There is really not much to the codex right now. A new release will granted come with a plethora of characters, but it needs more than that.
I say first of all it adds the 2 new assassin clades revealed in the Horus Heresy Nemisis book. Next, Inquisition factions would be cool. Like certain inquisitors grant certain things based on their faction.
As far as Grey Knights go, they are cool, but seem lacking. Terminators, Marines and Dreadnoughts? That's it. Purgation squads I guess, but they only have 2 weapons to choose from (which I will comment on momentarily). I don't know if jump troops should be added, but if they do get the storm raven gunship I wouldn't be surprised. I don't think scouts should (scouting daemonic worlds doesn't seem feasible) but the GK need some more character. Chaplains and Librarians would be a good start. A Chaplain does show up in Ben Counter's books. Some sort of new type of unit would be great. I could see Jetbikes happening (which would help fix the speed issue as well.)
4- WEAPONS! The GK have 4. Nemesis Force Weapon, Storm Bolter, Psycannon, and Incinerator. Lame. They need some sort of Melta type weapon, especially in the Mech dominating field of 5th edition. Some other new toys for them would be nice as well.
Basically, I can't see the Daemonhunters being a game changer. They should be slightly similar to Blood Angels I suppose. You know, in the way Space Wolves are similar to Space Marines. Maybe a second army that plays like BA will help shift the game away from mech.
I for one avidly await to see what they will do.