The comments below are adapted from a response to a posting by a friend on another blog asking what made for great film romances.
There are some great choices in these comments. I particularly concur with The Shop Around the Corner (although I prefer the Lubitsch version with Jimmy Stewart and Margaret Sullavan). The mismatched pair is often a good device in love stories, as we see in this film as well as in many great romantic novels (e.g., Pride and Prejudice).
I also like the suggestion of The Lion in Winter as an especially bitchy (but nevertheless heartfelt) love story. I would contrast it with Becket, featuring the same actor, in the same role, as a much younger man who is very much in love with his friend Thomas Becket. Another great bitchy but heartfelt romance is Two for the Road, with Albert Finney and Audrey Hepburn.
One thing I haven't seen anyone mention is sacrifice. Great love stories often include great sacrifice on the part of one or both of the lovers. In this vein, I consider Wings of Desire to be a great love story because Bruno Ganz gives up immortality to be with the woman with whom he falls in love, and he does so with no guarantee that she will even like him once she gets to know him.
Roman Holiday is another great love story that includes tremendous sacrifice (in this case Audrey Hepburn must sacrifice her love to duty) and also redemption, which is another important theme in many love stories. The character played by Gregory Peck is redeemed by his love for Audrey Hepburn.
And Brief Encounter is, of course, another great story of sacrificing love to duty.
One other device often found in love stories that I'd like to note is devotion, often of the selfless variety. A great example of this is Charlie Chaplin's character in City Lights. The Tramp has very little to recommend him to the flower girl other than his devotion. The expression on his face at the film's end has evoked tears from me more than once.