The use of Caucasian actors to portray the Chinese lead roles is not surprising. However, it is interesting to note Cheng's exaggerated, stoop-shouldered posture and shuffling gait, and his extreme meekness (really apparent in this line from early on in the film: "The sensitive yellow man shrinks in horror"). The portrayal of Cheng's "Chineseness" is stereotypical and rather derogatory to today's audience, but I suppose that during the era it was made, this was not an out-of-the-ordinary picture. The opium den, Buddhist temple, and various scenes of "life in China" would seem so hackneyed in a picture today, but I suppose they added an element of the exotic to Griffith's film. It's quite likely that many Americans viewing Broken Blossoms had never come into contact with an Asian-American, and relied on media to "show" them the nature of these "heathen Chinee".
The viewer is meant to sympathize with Cheng and his unrequited, pure love for Lucy Burrows. Lucy and Cheng are similar in many ways -- Lucy's cowering and Cheng's hunched shoulders, Lucy's abuse from her father and Cheng's abuse from white soldiers, and their inability to be happy in their environment. But by being compared to Lucy, Cheng is emasculated -- he is weak and effeminate, in contrast to Battling Burrow's raging, testosterone fueled masculinity. The audience, however, is not meant to like the alcoholic Burrows. It seems rather shocking when, at the end of the movie, Cheng defeats Burrows. Cheng's suicide can be read in different ways -- an act of honor, of heartbreak, or of cowardice.
Race, in Broken Blossoms, is really a confusing concept to me. The film portrays a taboo interracial relationship, between a white female and Asian male, no less! The threat to white female purity was certainly a major issue for many nativist Americans during the time that this film was made. Also, the racist characters in the film (Burrows, the soldiers, etc) are generally cast in a very unflattering light -- they are uncouth and unlikable. Yes, this film is still a problematic portrayal of Asians, but for its time, I think that it is actually somewhat sympathetic. What was Griffths' intended message? I'm still not sure.