Player's Handbook 2 had significantly less issues than Player's Handbook 1. The V-class structure was nowhere to be seen, and the Striker mechanics were different. But the defender in the book, the Warden, left a bit to desired, and the Barbarian was a bit lackluster...
AvengerThis is a nit picky change, but Avengers in my version get a free bonus Domain feat from Divine Power that matches one of their deity's domains. It's just a nice way to further customize the divine classes, and isn't strictly necessary. But it's fun. Oh, and Avengers us their Wisdom to make Melee Basic Attacks.
BarbarianBarbarians' defenses are... well, too low seems redundant, but it's true. Giving them higher HP doesn't help the fact that nearly every shot that gets thrown at them hits, and hard at that. There are two ideas that I have to help with that: up the Barbarian's AC and punish people for hitting the Barbarian.
Presence of the Savage
If you're wearing light armor or no armor (cloth) you may use either your Constitution or Charisma modifier instead of Dexterity or Intelligence to AC.
Whenever you are hit by a melee attack the attacker is dealt damage equal to either your Constitution or your Charisma modifier+3. At level 11 this damage becomes 6+Constitution or Charisma, at level 21 it becomes 9+Constitution or Charisma modifier damage. This rule has not been playtested. I'm pretty certain it'll work out pretty well, given that it's just the melee attacks and that'll make people dance away from the barbarian and, given that the Barbarian can get places pretty easily that means you give him another charge!
Whenever you charge you add either your Constitution, Dexterity, or Charisma modifier to your damage roll. This increases to mod+3 at 11th level, mod+6 at 21st level. You may also use any At-Will, Encounter, or Daily powers on a charge.
God, that looks terrifying. The barbarian is now a charging engine of destruction that'll mow down anything in his path. Again, this stuff isn't playtested, so please try it out and let me know what happens. I look forward to it!
Druids' only real problem lies in their inability to deal damage while in Beast form, so augment all their damage a step as done in the Player's Handbook 1.
As a matter of fact, I think the Invoker's powers should be Charisma-based, not Wisdom-based. A mouthpiece of the gods that isn't Charisma-based? Riiiiiiight...
Wardens are so... there's so much potential there, and it's all in their marking powers, which are held back. They really shouldn't be Immediate reaction, because that means you can only use them once a round, which is very limiting. Move it to an Opportunity Action for both of them, and things will become more interesting on the battlefield. Promise.
These are the basic modifications I'd make to each of the classes. They make the classes more effective and flavorful, and give relevance to the tactical situations you'll run into in play.