Assuming they went with the Rageblood build for the Barb, expect the Barbarian to tear things apart. He is all about charging, and doing massive damage. So give him opportunities to charge (instead of blocking him). Also, several things happen for the barbarian on crits. However, the Barbarian is also a glass cannon; he is a melee striker who is going to take a lot of punishment because he is going to look like a big, nasty threat.
An interesting quirk about the Invoker is that the invoker has area affects which hurt enemies but help allies (similar to the 1st level Cleric power "Divine Glow"). So, offer an opportunity or two where the enemies and the PCs are in a nice little burst.
The Bard is not much different than a laser cleric, except that he can shift allies a bit more.
My only tip for the paladin is that, regardless of who the paladin Challenges, I'd have that target attack the toher PCs. That way, the Paladin's mark zings them for damage.
As far as the Avenger is concerned, they revolve around picking one guy, dragging him away from the combat, and duking it out with him, mono-e-monster. So I would emphasize that to the player; they are a God-empowered John Wayne.
I can't think of anything else, not having seen the adventure or the classes/builds.