I don't see how this just isn't flat-out better than the fighter at being a defender. You auto-mark everyone adjacent. You get the same interrupt when a marked foe attacks someone that's not you - except that you target Fort instead of AC, and give everyone CA vs the enemy. Or you can pin them down out to a range of 5 (I'm guessing you can't do both). Plus, you get 2 saves every round (possibly avoiding negative conditions flat-out) and have more hit points. As mentioned previously, the AC is a wash due to Guardian Might.
1) The fighter probably has better AC. Heavy Armor > Light Armor + Secondary Stat.
2) The fighter probably has better to-hit, due to Fighter Weapon Talent.
3) The fighter is able to punish enemies who shift away. The Warden gets to walk them slink out of reach - and while Warden's Grasp will bring them back and stop them from doing so in the following round, that means that against smart enemies, they are about half as effective at punishing foes as the fighter.
4) The fighter also gets to outright prevent enemies from moving away. That is powerful - especially against solos who will gladly soak up a hit to go prey on weaker party members.
All in all, I think the balance is a good one.
Defense-wise:
The Warden has the edge here, with more hp and probably only slightly less AC. Font of Life is really what pushes them over the edge - and honestly, is the only power I am remotely worried about in this class, since shrugging off conditions is extremely powerful in 4E.
Offense-wise:
The Fighter has the edge here. The Warden's retaliation is more potent, but also easier to avoid. The Fighter is very hard to escape, and their greater accuracy is not to be underestimated. The Fighter At-wills seem more potent, and more focused on damage - which again, helps encourage enemy's to attack them rather than ignore them. The Warden isn't completely left in the dust, though, due to how easy it is for them to place their marks - and Warden's Fury is usable with a reach weapon, unlike a Fighter's retaliation. The Warden giving up some defense to catch up on offense, on the other hand, seems like even more confirmation of the balance between the two.