[mdaniel]

I had to look up "didactic" to remember what it meant, and I learned that it has a negative connotation (according to this one dictionary, at least)

http://i.word.com/idictionary/didactic

[zem]

"autodidactic" is "autodidact" + "-ic" rather than "auto-" + "didactic"; this is not a pedantic distinction because "autodidact" has mostly positive connotations, whereas "didactic", as you note, often has connotations of unwanted moralising.

[mdaniel]

English drives me crazy and it's my native tongue

[zem]

the same features make it a wonderfully fun language though :) my favourite example of how building up a word differently changes its meaning is inflammable (inflame + -able, rather than in- + flammable)