RE: special weaknesses...
Presuming that the party has the capacity to run away, and that there is a relativly uncomplicated manner in which to achieve the conditions or items necessary to defeat the ghost, then that's great.
If the ghost say, reduces movement speed, dazes, or stuns, can teleport, or locks the players in a room, then yeah, it's a bad idea. If the ghost is blocking "important route X" and the players have to backtrack several sessions worth of traveling, then it's a bad idea. Unless you made it abundantly clear that X path could not be crossed without X ghost-defeating-item, in which case, it's their own fault.
While "kryptonites" are cool for super-baddies, say, The Ghost King of the Haunted Keep, they are annoying and cumbersome for average mundane encounters. Bogging down encounters with swarm-like mechanics makes a game boring and most importantly, makes your players annoyed. If you intend to do this, I suggest a "ghost saying pack" similar to an adventurer's kit, that includes a small amount of whatever it takes to kill most mundane ghosts. However, once they've traveled to Far Away Kingdom, the ghosts are different, and they need a different pack.
Bogging down games, making encounters that lock up easily, repitious gameplay, these are things that should be avoided. No, I'm not saying make it easy, I'm saying that games are not about players defeating the GM or GMs defeating the players. Pitting GMs against players is just annoying, encounters should challenge players to use their in-game and IRL skills. Not simply lock players down because their opponents have some special weakness that's only found on some special island that grows from the hidden special tree on the unique and special day that nobody knows about.
Edit: From a mythos standpoint, ghosts are generally comprised of some substance to affect the material world(and be affected by it). Either they are posessing something or someone, or they are composed of some kind of ghostly material, ie: ectoplasm. When whatever is tethering them to the world is destroyed, be it the lamp they possessed, the golem they inhabit, or the ectoplasm they're made of, then they too are vanquised, or at least run off until they can re-posess something or generate new ectoplasm.
From a very logical standpoint, a true ghost would immedately posess something else or be as you suggest, completly unhittable, but able to materialize it's claws long enough to harm you. "true ghosts" like these don't exist for the sheer reason that they cannot be vanquised, and must be run away from(or killed by). On rare occassions, they can be "destroyed" though holy or unholy means, assuming your party lacks a holy/unholy character, this means you are facing an invincible foe. And for Generic Orc Ghost minion, this is simply an incredibly annoying event that usually only exists to allow GMs to laugh at their players.