Is the phrase "recall that..." an abuse? I love how in some proofs delivered formally or informally, you would always encounter the phrase "recall that".
The irony here is that you precisely do not recall this notion or formula e.g. the polar form of the laplacian, that's why it needed to be brought up again.
What's the opinion from the math community on this?
 A: It depends. A lot of the time I see this, it's an ugly thing that I've seen but didn't remember the specifics. Your example about the Laplacian in polar coordinates is a good one. In these cases I read it as "hopefully you've seen this; here's a refresher; if you haven't seen it at all, go look it up elsewhere, because we won't be spending much time on it." 
So this isn't so bad necessarily. Compare it to the extreme of terseness, which is to simply use the result in the middle of a proof without singling it out as something to be recalled. Yet we do this all the time with results that become routine, even when they are actually fairly sophisticated results. (Holder's inequality, for instance.) 
So there is some balance to be struck. Some things need explanation in the text. Others need little to no explanation at all, being assumed as background material. In the middle, there are things that some readers might not already know, which should be singled out to avoid confusion, but that enough readers will already know that you shouldn't waste too much time on it.
The problem is when something which the reader thinks is in the first category gets lumped into the third category. Rarely does something the reader thinks is in the first category get lumped into the second category.
All of these things become much less irritating if a reference to a less advanced work is provided.
