Actually, the 15 Con requirement on Treewalker is HARSH. Most of the characters who are interested in it won't have that kind of toughness (the points came out of his Wisdom score for Bud).
EDIT: Having chatted again with the player: 15 of a stat is usually only used in build-defining feats (TWF is a notable example), and Treewalker, while neat, is not anywhere near this metric.