What's wrong with 'preachy' is just the oversimplifying of complex ideas. There's nothing wrong that one tries to convince people of a point of view. Nothing wrong that one has many strong opinions. Nothing wrong that one really tries to tackle complex concepts.
What's wrong with pretentious is ending up being too hard to understand for no good reason at all. And what is a good reason is up for debate. In fiction one may be going for a subtle effect. Such as Hal Duncan's Vellum has a mystifying effect which a reviewer complained about as hard to understand and ultimately just pretentious. But it's intent is a somewhat subtle effect.
Preachy is a dumb word I've never used to describe anything. I'd just say a gross oversimplification or something like that. Otherwise you're left with the idea that any attempt to state one's point of view about any ideas/concepts that aren't universally agreed upon is possibly preachy.
I think I need to quit thinking of such things. Actually had in mind a graph. heh. Was thinking a quality of language and seeming complexity of thought that's too complex for the ideas ends up to the upper left on the graph (pretentious) and a level of thought/language that's too crude for the complexity of ideas end up on the lower right (preachy). And a slope of the perfect writing which is neither preachy nor pretentious goes from the lower left to upper right.
But then thought of endless complications. Writing I think both preachy and pretentious, etc. Oh well. The main problem is that 'preachy' is just a dumb word. Secondly the parameters of such a graph are just too crude.
Not the best use of my time, lol.
...the graph would work better if that which fell to the lower right was deemed oversimplifications instead of preachy. Otherwise... I don't know. Maybe that's halfway useful. To me and me alone. The harder the concepts you attempt to tackle the harder it is to not fall into oversimplifications. The word 'preachy' is just no good though. It's not necessarily a bad thing at all to be preachy although certainly some people don't want to hear it.
It is possible though to manage to be pretentious while also oversimplifying as each new sentence is a new chance to mess up in a new way.