Spoiler
Both Roland of Gilead and Thomas Covenant are missing the same two fingers of the same hand.
The Dark Tower and the Arch of Time are very similar and The Crimson King is trying to destroy the Tower while Lord Foul is trying to destroy the Arch.
People from "our" world are transported to both worlds in nearly the same way; through dying in one world or doors in time (falls?) Also, Thinnies and Falls seem to be nearly the same thing.
Although I haven't seen ka-tet or any similar phrase mentioned in the Covenant books it seem Linden has her own sort of ka-tet.
Lord Foul and The Crimson King are very similar, although I doubt Foul will turn in to a spider (hopefully)