One is the story. Of course it features a love-story but the movie takes an original approach and takes a couple of nice turns with the love-story. Who is falling in love with who always remains a surprise in the movie.
Another reason is an obvious one; Frank Sinatra and Gene Kelly. Not only are they great genre actors but they’re also great together. Especially Kelly delivers one fine performance. The female cast is also impressive by the way, with Esther Williams as the female lead.
Another important reason why this movie is so great is the cheerfulness of the whole movie. The songs are cheerful and never really too sappy or just plain awful. The movie has some great comedy moments and is also definitely helped by its fun main characters.
The movie sort of pays a great homage to the early days of baseball in America. The settings and premise work out fine for the genre and make the otherwise still somewhat formulaic storyline still come across as original.
The production values are also good and everything combined makes sure that this is a good pleasant bright musical comedy that is perfect to kill some time with and have a good time with at the same time.