There is an API incompatibility in log4j-over-slf4j 1.7.2 (and earlier versions) with the original Log4J 1.2.x: the class org.apache.log4j.Category is missing the method "public void setLevel(org.apache.log4j.Level)". Even if it would do nothing in log4j-over-slf4j (as does getLevel(), which is not missing), it should be present, IMHO.

I have not checked whether there are more incompatibilities, I have just noticed it when trying to compile one of our projects with log4j-over-slf4j.