Different level of toxicity between smoke
I am not sure, if any comparisions have been made with other plant leaves, but unless these contain some pharmacologic active substances, I doubt that they are smoked widely.
There are comparisions between normal smoking and the smoking of marijuana and the short version is: Both are harmful. It depends on the temperature at which they are burned, how they where grown and so on. For smoking marijuana it also depends if you mix it with tobacco...
This fact sheet from the University of Washington lists a number of points on marijuana together with proper references.
Then there are some articles about smoking and marijuana in comparision:


A Comparison of Mainstream and Sidestream Marijuana and Tobacco
Cigarette Smoke Produced under Two Machine Smoking Conditions
On the Carcinogenicity of Marijuana Smoke
A possible chemical basis for the higher mutagenicity of marijuana
smoke as compared to tobacco smoke
Is frequent marijuana smoking harmful to health?