General Synonyms:
* Instructive
* Informative
* Educational
* Preachy (often has a negative connotation)
* Expository
* Didactical (more formal)
* Pedagogical (focuses on teaching)
More Specific Synonyms:
* Moralistic (focuses on teaching moral lessons)
* Dogmatic (asserts a rigid belief system)
* Propagandist (aims to influence opinion)
* Paternalistic (implies a superior-to-inferior relationship in teaching)
The best synonym for "didactic" will depend on the context in which you're using it.