Aren't Vulcans and Romulans supposed to be physiologically identical?
I believe that book said there was a very minor difference between the two races, something barely detectable. There is the issue of the Romulan forehead. Some Romulans don't seem to have it, and Spock was able to impersonate a Romulan on their homeworld without it. No Vulcans are known to have the Romulan forehead, so that does seem to be a physical difference.
I think that Romulans aren't telepathic, which has been a plot point in some novels, including details on Saavik's background, that she was born in a breeding camp where captive Vulcans were being forced to breed with Romulans to see if their hybrid children were telepathic.
I think there's debate but no clear answer on what causes Pon Farr. There's no evidence that Romulans go thru Pon Farr, but the controversy is whether Vulcan Pon Farr is caused by their emotional repression or if it's a biological mutation.