|Clerics of the Diety of Demolition Work|
The Turn Undead ability is a major component of the old school Cleric. It's a very specific ability that only Clerics receive. Deities really hate Undead for some reason, which is odd because if ALL Deities hated Undead why is there so much Undead left? Possibly the Turn Undead ability is an extrapolation of a holy man's ability to perform exorcisms and shoo nasty spirits away.
My take-away from this is that old school versions of D&D expect the appearance of Undead to be as likely as Player Character's getting wounded and requiring healing. Any campaign world you run using an old school version/variant of D&D assumes the existence of hordes of skeletons, ghouls and ghosts roaming about. The existence of Undead is built into the very core of one of the main classes, and therefore the game.
I don't use Undead in my Campaigns (often). Their not a group of beings that appeal to me. I prefer things that are monstrous due to an overabundance of life. I prefer slime and tentacles to bones and dust.
This begs the question: Does a Cleric playing in my campaign thus have a fundamental disadvantage by being denied the use of one of their abilities? I'm not sure of the exact trade off, but Clerics are worse at something, somewhere along the line because they can use Turn Undead.
I feel that, yes, in a Campaign without Undead, a Cleric is disadvantaged. They are denied one of their core abilities.
This got me thinking as to how to replace the Turn Undead ability in a Undead-free campaign*.
This is easier to do when playing a game using a "HD amount of Undead" turning table, rather than a "Specific Undead Creature" turning table. In this case you can just replace the "HD amount of Undead" with "HD amount of whatever Thing the Cleric's Deity really hates". When playing a game that uses a "Specific Undead Creature" turning table, a bit more work is required to find equivalent beings. Though, you could just ditch that system and move to a HD based Turning table.
Replacing "Turn Undead" to "Turn something the Cleric's Deity hates" allows customization to the relatively rigid old school D&D cleric. Plus, it may make the oft neglected Cleric class a little more appealing to players.It also gives greater definition to the Deities at play in a campaign.
If you have your own campaign Pantheon it's pretty easy to work out what specific thing each Deity REALLY hates. If you allow a Cleric to make up their own deity, or bring one from another campaign setting, its probably worth spending a little bit of time defining exactly what that deity REALLY hates.
Replacing "Turn Undead" with "Turn Something the Cleric's Deity Hates" has a lot of scope to be either really overpowered, or be so specific as to be useless. Though, this is also the case with Vanilla Turn Undead.
Evil Deity of Alcoholic Beverages and Inebriation; Cleric has the ability to Turn Sober Pub Patrons. Sober people flee from a site of alcohol abuse. (This is one of the ones that are so specific as to be useless.)
Good Deity of Order; Cleric has the ability to Turn Aberrations. These could be things with too many arms or legs, mutated things, people who believe Anarchism is the best form of government (Not to say Anarchists are aberrations, but they are by definition at least some what opposed to Order).
Neutral God of Nature: Cleric has the ability to Turn Technology. This would be things like Mecha-Golems, Robots and Hi-Tech Weaponry. Could also turn (cause to malfunction) things like Looms and Mills.
The alternative route would be finding a group of creatures analogous to Undead in your campaign (as they were perceived in old school D&D [as in so prevalent a class needs to be allocated an ability with whole half page table devoted to combating them]), but I feel that wouldn't be as entertaining as having a Cleric who has the ability to send a Loom running from the room in fear.
*Or free enough for a Cleric to have cause for complaint.