Yeah, and it doesn't take much to convey that this is a conversation between peers. A couple simple changes I get a lot of mileage out of. Where once I'd have written:

"Do [X]"

Now I write:

"I see [problem Y], consider making [change X] to improve it"

If the reviewee agrees then the change is easy and straightforward to make, but if they're unconvinced then the phrasing invites a dialog.

Or if I think I see a bug, I'll phrase it like:

"I think there's a bug here, how does this method behave if foo is null and bar is the empty string? I think we'd throw a null pointer error. Recommend adding a test for that case"

Clear, actionable, refutable