I had similar problems in my group from a couple of people who had come over from 3.x edition. It has more to do with the premise of each edition, with 3.x being based around "guitar heroes" and 4th being based around "rock band" (stay with me, it makes sense).
In 3.x, you were a bunch of people fighting in the same place. Every character was built for individual glory. Basically, you were a band of 4 lead guitars, all simultaneously trying to be the star.
In 4th, you are all part of a group, each with a role to play. Each character is built to add that function to a group that, together, is a star unit. Basically, you have a band that functions so long as everybody is trying to make everybody else work well.
So, in 4th, if your striker wants to take down someone in a single hit, then s/he needs to work with the rest of the party to get the setup and bonuses to make it all happen, with the defender occupying the enemy, the leader buffing the striker, another striker taking the monster down a notch, and/or a controller debuffing the enemy, so the "assassin" striker can use his/her most powerful attack with certainty that it will land and great bonuses to the damage dealt.
Of course, forcing a playstyle on the players is not nessasarily fun for the DM or the players. So as an alternative, I'll go with what some others have suggested.
Consider using Elite Minions...
I started using Tougher Minions, Elite Minions, or what I call Minionized Monsters a while back, with great success.
I basically take a monster, give it vulnerable=bloodied value once per turn to any attack, and give it 1/3 the XP value. Also, it is indistinguishable from the non-minionized monsters in the battle.
If you manage to deal damage to it equal to it's bloodied value, then it dies because of the vulnerability. Then, I describe the amazing actions the players are taking that is causing this kind of damage. It makes the strikers in the group happy that they can occasionally one-hit a monster, and it allows for much tenser situations as they players face down what looks to be a L+4 or L+5 challenge.