I know Ents dislike axes, but this review is a hatchet job.
Leaflock is the worst of the Ents. There is no Ent card that I would hold back on so I could play Leaflock. What's that? He can have 4 for 3 cost, you say? While that's true, that simply isn't enough to redeem him.
Let's compare him to the most vanilla of the Ents, Wandering Ent, a 2/2/2/3, 2-cost ally. Both enter play exhausted, cannot have restricted attachments. Both have identical , , and . The differences lie in their willpower and their cost. If you have the misfortune of drawing Leaflock early, before you have Ents to stack damage on, he's a more expensive Wandering Ent with less willpower. That's no good. So obviously you want him late, right? Well...
Let's see at what a good board state for Ents looks like. Besides other unique Ents such as Quickbeam and Treebeard, there's a nice combo between Derndingle Warriors, Booming Ents, and Wellinghall Preservers. The Wellinghall Preservers take care of questing, the Derndingle Warriors, possibly with an Ent Draught for really high attack enemies, take care of defending, and the Booming Ents are very good at tearing enemies apart. If there's no source of direct damage such as archery, then damage will only reliably be placed on Derndingle Warriors, Quickbeam, and Beechbone. But you don't want the damage on the Warriors to stick around, you want the Preservers to heal that damage during the refresh phase (or by readying through other means such as Narya) so that your defensive solution remains solid. The Booming Ents don't have a problem with this; the damage stick around long enough for them to attack. But by the time you reach the quest phase, that damage should be healed, and Leaflock will be left a more expensive Wandering Ent.
So that leaves quests where there is archery and other direct damage, and you can spread ticks of damage to your Wellinghall Preservers, Booming Ents, and Wandering Ents, causing your Booming Ents to rival the ferocity of Boromir holding a torch. But in those quests you simply need healing all the more, so Wellinghall Preservers are the priority. By the time you have enough of them and other prioritized Ents out to have resources for Leaflock, you simply don't need the extra willpower. To really rival the usefulness of the other Ent cards, he should probably start with some willpower, maybe 2, and have a higher willpower limit, such as 5 total willpower, to give you a dilemma in which Ent to play.
In summary, he's a more expensive version of the pile of stats that is Wandering Ent in the early game, and even when he does get all 4, he's simply not needed. Unfortunate for one of the eldest surviving Ents, but in a way thematic given the fact he's grown so "tree-ish" lately.