A lot of similar elements between two dramas such as revange, mafia, police, undercover etc. Both dramas are very dark.

Mafia gets involved with the law;
in Vincenzo it's not that direct, and there are some bright moments

The plot is exactly the same (Not sure if they're both remakes of the same drama or just based on the same manga/etc).

In both the girl tells a story of her high school, about her life, friends, how she fell in love.
Both of movies/drama has those similar 'feelings' because action takes place in 1990s :D

both say stories about japanese occupation in Korea, fighting about freedom, friendship, war, love...
and both of them have amazing soundtracks

The same male lead, both with supernatural elements and a bit of humor ;) And we're going to have romance there :D

Both are dark, mysterious dramas, with plot turning upside-down; great cast, more 'adult dramas'

Comedies mixed with melodramas with great cast, showing really great romance

Classic rich-guy-poor-girl dramas with one girl and 'flower boys' living together in one house

In both dramas we have the same male lead.
The characters are that kind of guys that girls wish to have and they treat girls 'good'.

The same male lead in both dramas playing character which was accused of a crime he didn't commit.

Both are about ex-couples making films together and their 'reunion'.