Watchlists can now be sorted into several orders. Manual control and categorization still in the works.Francois-Alexandre Perreault commented
This would be such an helpful feature. At the moment Letterboxd is the best way to manage everything movie related but as mentioned by the OP, once you grow a massive watchlist you cannot really find what you are looking for easily.
As of right now, I have to use private lists to work around this.