Locks, branches, and character specialization
A skill tree is a directed acyclic graph of abilities: you unlock left-hand spells by spending 5 points in the fire tree, unlock right-hand spells by spending 5 points in frost, and unlock the capstone (meteor shower) by unlocking both branches. This gating forces players to specialize: you cannot have everything at once. A player who spreads points across all trees gets generic skills everywhere. A player who focuses on fire + frost unlocks synergistic combos (chained spells that deal bonus damage when both trees are active). Depth comes from the interplay between lock dependencies and player choice.
Respec gates and the cost of flexibility
Most games let you respec (reallocate points), but at a cost: rare currency, a long cooldown, or NPCs that charge gold. This cost exists to prevent players from being optimal at everything (trivializing the tree) and to make builds feel meaningful. If respec is free, every player is the same optimal build. If respec costs 10,000 gold and you have 50,000, you can try different builds; if it costs 1,000,000, you are locked in for your playthrough. A good respec cost is high enough to discourage constant switching but low enough that a player can fix mistakes. This is a dial that controls build diversity and experimentation.