While I like most of the changes that Pathfinder made to 3.5, I must admit that I was disappointed that they didn't eliminate what is, IMO, one of the worst elements of the magic system, the Hit Die limit on certain spells. Cause Fear, Sleep, and several other spells have a limit on how many HD they can affect, meaning that these spells eventually become completely useless. I think this is an absolutely terrible mechanic. A spell should never become useless! Worse, in some cases this is used to justify a spell being quite overpowered (Sleep, I'm looking at YOU), with the rationale that it's okay for it to be overpowered from level 1 to 4 because after level 5 it's useless. This is just terrible game design.
And looking at Cause Fear, for example, why is the 5 HD limit even needed? It affects a single target and only lasts for 1d4 rounds. I can't imagine this spell being overpowered even if you could attempt to use it on a level 20 monster. This is even worse for its successor spell, Scare, since it is only useful for a whopping 2 levels before becoming ineffective (or only 1 level for a sorcerer). Fear, thankfully, doesn't have this problem. Granted, it's a 4th level spell, but it also affects a large area and inflicts a worse degree of fear than Cause Fear and Scare do. These spells really would be just fine with their HD limit removed.
So, would it really kill the game to remove the HD limit on these spells? Naturally, a couple of them would need to be adjusted to balance this change, like Sleep and Deep Slumber, though most of these spells would probably not need to be changed in any other way. Pathfinder did change one spell that became useless at higher levels in 3rd edition - Dispel Magic, and I welcomed that change. They really should have gone all the way and made sure that every spell has some relevance no matter what level you are.