As many probably are already aware, Lamentations of the Flame Princess makes turn undead a spell rather than a cleric per-encounter class ability. In general, I think this is an attractive change, as turn undead can totally obviate certain challenges in a way that is not so engaging (especially at higher level). However, I think there are some problems with the LotFP implementation.
Specifically, though a spell slot must be dedicated to turn undead, the player must still resort to rolling on a traditional turning table. This is both mechanically awkward (as it requires a table lookup or extra data copied to the character sheet) and somewhat against the spirit of spell slots. Though it is true that some “slotted” spells can also be avoided based on a target saving throw, in general I prefer when players deciding to dedicate some resources results in at least some effect. As written, the LotFP turn undead spell occupies a spell slot and may still prove useless if the 2d6 roll is unlucky.
I still like the basic idea though, and I can think of two ways to adjust the spell. In the first method, you keep the turning table, but allow as many turn attempts as desired (though no more than one per encounter) for the duration of the spell (which is given as 1d4+2 turns; see Rules & Magic, page 146), rather than being terminated by failing a turning roll. I might also prefer to base the duration on cleric level, perhaps 1d6+level exploration turns rather that 1d4+2. The second method, presented below, bases the strength of the spell on the level of spell slot used (or the level of the cleric divided by two, rounded up if not using spell levels).
Cleric spell of any level, range is 30′ or as light source, whichever is less.
This spell may be prepared in any available spell slot. The power of the spell is proportional to the level of slot used. Undead of HD less than or equal to the level of the slot are automatically turned, and those with HD equal to half that level (rounded down) are destroyed (reduced to ash and blown away as by a strong wind). While turned, undead will not enter the light, but instead creep around its edges. All undead within the range are affected. Turned undead may be kept at bay indefinitely with concentration. Attacking turned undead or pressing them aggressively (such as into a corner or over a precipice) ends the effect.