I agree that we should avoid slang (popular expressions that change as often as fashions do!), but there are expressions (idioms, colloquialisms) that have entered every language's popular vocabulary and should be part of any language learner's "homework"! That's not to say that we shouldn't be mindful of the variety of languages spoken here (we do, in fact, have some board visitors whose first language is neither English nor Spanish!), of course!
I hope board users will ALL feel comfortable asking for help when an expression is unfamiliar...and the number of threads that have been dedicated to language issues (slang, local expressions, differences between Spanish in Spain and in the Americas, etc.) certainly suggests that members like to talk about "how people talk"!
Andres, I think "zero in on" comes from using military targeting equipment (and the expression is not a new one), and "man about town" is commonly used to refer to a person who knows a place well and moves about in it smoothly.
And we ALL should know what "brave potatoes" means!