Fear-based spells can be a lot of fun. They offer players certain options--scaring opponents off rather than having to kill them, disabling or hindering opponents to make them less effective, etc. When they work, they add extra color or flavor to combat instead of just hacking away at enemies.
The problem is that certain lower-level fear spells have hit dice restrictions:
Cause Fear - one living creature with 5 HD or less.
Scare - one living creature/three levels, each must have 5 HD or less.
To me, these hit dice restrictions seem arbitrary and unnecessary. The spells already have a DC. Why add second layer of hit dice restriction? (It might even be a third layer if the monster has spell resistence.) Since these are low-level spells, the DCs end up being lower than those of other spells. High level monsters will generally have better saves than low level monsters, so it already works out that higher-level monsters are less likely to be affected.
Consider the following: a first-level wizard can use Cause Fear to scare monster with five hit dice, but a 20th-level wizard can't use Cause Fear against a monster with six hit dice. Any six hit dice monster that isn't scared of a 20th-level wizard is an idiot. This rule doesn't make any sense.
I would be fine with implementing a restriction based on the caster's level/hit dice. For example, we could make it so that Cause Fear is not effective against monsters that are more than three hit dice higher than the caster.
As it currently works, the spell caster reaches a certain point in level progression and discovers that these two spells have suddenly become completely worthless. It's fine if these low-level spells seem less effective over time, but it's another thing if you're only facing monsters with 6 or more hit dice and they literally can't be affected by the spell. Also consider that the spells I listed appear both on the bard and sorcerer spell lists--spell casters with limited lists of spells known. It is unfortunate that these spells have such strict restrictions.
