I'm a little disappointed with the handling of the Romulans in the series. Specifically, that we find out what they look like as the audience, but the heroes remain unaware for canonical reasons. They could have been the Faceless Enemy for the entire series.
I know it was easier for storytelling reasons to show the Romulans to the audience, but with their prominent appearances in Season 4 we learn way too much about their appearance, personality, and even their war plans. But since the main characters never see them, that's supposed to be okay.
We still could have seen their ships, there still could have been threatening conversations via subspace radio. But we didn't really need to see what was happening on their ships and planet.
I'm sure this has been kicked around before, but I didn't see the entire series until about 4 months ago.