Both dramas involve incest (supposed in one, real in the other).

Both stories have a love triangle and in both dramas there's at least one character who becomes obsessive/delusional.

They're different (both the plots and the leads' relationship status) but, at the same time, I've found them similar (especially at SL's beginning; while watching I often thought about Hirugao).
Differences: the ending and the mood (they're both realistic but SL is lighter whereas Hirugao is more emotional, maybe because the latter is longer).

Even if the plots and the main characters are different, some characters appear in both dramas (in Faith they're young, while in The Great Seer they're older).
In both dramas there are supernatural powers (used to fight and to time-travel in the first, used for divination/geomancy in the second).

Both dramas' female lead falls in love with the most popular guy at school (the male lead).
One of the various differences: the male leads' personality (in PK he's more rational).

Although they're completely different, the part (in Moon River) about the election of the school's spokeperson, reminded me of City Hall: both dramas' female lead wasn't interested in politics, but was convinced to participate in the election.

They have one thing in common: both dramas have a mother that, after losing her son (for different reasons), take care of another child raising him as her own.

Both dramas' male leads are computer genius and CEO/founders of an IT firm.