Lets see, what makes a fighter a good defender?
- The ability to absorb attacks. A fighter has a lot of HP and a lot of healing surges to use per a day. To improve the fighter's ability to absorb attacks, he gain access to powers like Boundless Endurance, Iron Warrior and No Surrender.
- The ability to draw enemies near him and away from his allies. A fighter gets Combat Challenge and Combat Superiority class features. To improve his ability to draw enemies near him, he gains access to powers like Come and Get It, Harrying Assault and Warrior's Urging.
If you improve only one of those, you'll end up with either a character that is hard to kill but doesn't defend his allies, or a character that attracts all the enemies to him but is killed easily.
In our game, we have a fighter, a ranger, a rogue and a wizard. I have noticed that the fighter neither kills enemies as easily as the strikers, nor does he deal with the many minions better than the wizard. So I think way to make a fighter really shine is to make him a good defender, and then take advantage of the fact that your character is going to be surrounded by enemies to use power like Sweeping Blow, Unyielding Avalanche and Cruel Reaper.
Notice that in order to be a good defender you don't have to have the best AC in the party.
I haven't seen a Paladin character being played in 4E, but I can see that he gets more healing powers than the fighter, in exchange for being worse at drawing enemies to him.