Here's my take on the Venerable Lord Kroak.
I would rank lord kroak as the second best special character (after Tetto) in the new book. I think he is really great at what he does. As back in the good old days kroak is prob best in big games where you can take a second slann to give you the utility that kroak lacks. you need to make sure your list is not reliant any any buffs other than Wildthang which you can get from the skink priests. But to kroak!
1) He makes your temple guard unit flippin awesome! Unbreakable and -1 to hit from ranged and combat attacks. Throw him in a horde of temple guard and i would give it a banner for some speed (Jaguar

) to help it not get chaffed to oblivion.
2) his spell aint too shabby 2d6 s4 hits is nice, and if your fighting undead or deamons GG. But of course the best way to use this is through your skink priests, particularly one with a cloak of feathers! It might even be worth taking a trogolodon so you have a more survivable vassal to throw into the middle of your enemies lines. The beauty of this is that you should be able to be hitting half your opponents army with 2d6 s4 hits by turn 1.
3) another factor going toward his awesomeness at making OP temple guard unit is he can't miscat!!!! no more blowing up half your temple guard unit with miscasts not to mention.....no miscasts is epic......
4) this dude doesn't die! T5 6W 3+ WARD SAVE. Other Than flaming magic dwarf cannon balls -_- this dude is pretty unkillable so no need to worry about charging the unit in as fast as you can!
5) Finally, the kicker. HE IS 400 POINTS. 400!!!! Lord kroak is apparently our cheap choice now.....who saw that coming.....
I think the ideal place for kroak is in a 800 point temple guard deathstar charging down the middle of the board in a 3k+ game. Given that, at 400 points he is going to do just fine in 2400 pts!
Pros: makes beefcake temple guard units. Serious damage potential if enemy does not deal with you vassals. Cheap!!!
Cons: 1 trick pony. If you let your vassals die your going to have trouble doing serious damage with him. Your army will lack the massive utility that we all got so used to from 7th edition (he can't make your I1 lizards move 20 inches a turn an always strike first =/ )
P.S. : can we discuss how mad I am that GW is trying to tell me Skink Oracles on Trogolodons discovered the Golden Pyramid of the Turtle Islands instead of Lord Kroaks head skink priest Ten-Zlati....... they be trollin on my fluff yo!