Phrases to avoid:
S/he wasn't stupid.I've had to resist the temptation to use this one, quite recently. But really, the character's actions will show whether or not they are stupid, which makes the sentence redundant.
S/he wasn't naive.Similarly for this one. Again, the next sentence will presumably describe in more detail why s/he wasn't naive, in which case that sentence isn't needed, or it won't, in which case you've got a problem because you're telling the reader something which you aren't willing to back up with character-action.(*)
S/he wasn't the type to...Wishy-washy phrasing. Do, or do not, there is no "type". Well, unless one is talking about personality types but that isn't usually how a sentence like that is used.
(*) I was originally going to say "show, don't tell", but now I'm afraid to use that phrase since
altariel (I think it was her) jumped on me (well, on my icon that had that on it) because "it's all telling!"... that is, it's all storytelling.
I think there's two aspects to why "telling" is commonly frowned on.
1) Merely a matter of style. Narrative-heavy stories with strong authorial voice are terribly unfashionable nowadays (while they were extremely common a ways back). A more cinematic style is what is "in". For this, I can understand why the annoyance arises about the "no telling" stuff.
2) Less a matter of style, and more a matter of respecting the intelligence of your reader. To "show" gives the reader a chance to draw their own conclusions, rather than telling them what they ought to think. "Telling" can also be used to make assertions without evidence; for example, the author tells the reader that the character is clever, but the character keeps on acting stupidly. (I vaguely recall someone mentioning an example of this kind of thing from
Twilight, but I can't remember what it was)