A couple of thoughts and observations:
First, I played a very brash and preening Dragonborn who specialized in fighter Close Burst exploits (Come and Get it, Action Point, Thicket of Blades?), and he also took Potent Challenge. Marking massive groups of people (thank you, Dread Reaper) and giving them -3 to hit anyone else is Defending, no matter what you might think of it. If I was a Leader and gave you +3 to all your defenses except vs area attacks, you'd think that was a pretty good defense boost, no?
Second, I think the Warden is getting sold a bit short here. His mark, admittedly, is not the greatest. However, look at Warden Powers and Feats. There are Warden powers which make the entire area around the Warden difficult terrain (no shifting, for most enemies). This means that you HAVE to move to get away from him. There's a warden feat which makes his opportunity attacks Slow the person they hit. You've got a huge ring of people who struggle to get away from you (Hey, you're slowed in Difficult Terrain! You get to move 1!). There are plenty of Warden powers which immoblize in a burst, or knock prone in a burst. In addition, there are Warden powers which give out healing whenever someone attacks the Warden. All of these things work as a defender, but NOT in the same way that a fighter's mark/CC/CS do. In addition, the effortless marking of Wardens (while limited in range) is much, much easier to accomplish than that of a fighter or paladin.
Finally, the case might be that your problem is with your group, and not your defender. If the wizard specializes in area attacks, and everyone else closes with the enemy in melee, is that the wizard not being effective, or the tactics of the party not being compatible with the wizard? Some groups, to my mind, don't "need" a defender. They don't have a particularly vulnerable member (or have one who is highly mobile). They don't use a lot of ranged attacks, but prefer melee and close ones. They would prefer a short fight (4-8 rounds) and a bunch of healing afterwards to a long fight where they take less damage. Those groups aren't wrong, but they might not be rewarding for a 'traditional defender'. A striker-oriented Great Weapon Fighter might be a better match, or a Ardent Vow Paladin, or an aggressive Tempest fighter.