A special case of offensive and affected writing is the double negative:
The psychiatrist's principles were not dissimilar to Freud's.
It was an annoying noise, not unlike fingernails on a chalkboard.
Pompous writing like this is a crime against your audience. Replace phrases such as not dissimilar and not unlike with the more direct word similar. Otherwise, you force your readers to stumble, double back and then translate the double negative into plain English to figure out what you mean.
Never trip up your readers. Say what you want to say, and say it simply. And avoid crimes of pretension.