Yes, we’d like to do this, but it’s not a quick development job so we’ve not been able to prioritize it to date.Ataul Munim commented
yesss :D but with a modification, as two users who might have similar taste, but not seen the same movies would get 0% similar (Letterboxd has no basis for comparison).
modification: Only produce a score based on films which are in both users' watchlists, like +1 point (to the score) if they both saw the movie but neither clicked like, -1 point if one clicked like and the other didn't, and +2 if both clicked like.
It would mean that everyone has a similar notion of the like button - some people use it as a favourite, and others simply as a like (I'm guilty of both at random times).
The best we could offer is probably a search based on film name/year, but this would be pretty accurate for most films.