The depth of your audience's emotional investment in the central romance is directly proportionate to the size of the story's stakes.
This arguably way-obvious observation was brought home to me by seeing a current release that isn't a romantic comedy. The Adjustment Bureau is a romantic drama (sci-fi/thriller), but its featured couple, politician David and ballerina Elise (Matt Damon and Emily Blunt) have great chemistry, both on the screen and on the page (via screenwriter/director George Nolfi) - the kind of palpable, sparks-shooting attraction that a good rom-com couple should possess.
This is due both to the actors' charm and the script's adherence to basic principles (see the two Rom-Com Truisms below); we believe how things go on their "cute meet" because David and Elise are so clearly a great fit. Yet the reason we grow increasingly more involved in the fate of this couple has to do with what the story supplies as the obstacle to their happiness, the Thing That Threatens Their Union. It struck me, watching Bureau, that this is where a lot of today's rom-com specs and released movies alike go wrong.
Not wanting to be a Spoiler, I can't be as specific as I'd like to be, so let's just say this - you can buy into Bureau's premise or find it patently absurd, but the movie gets one thing absolutely right: It tells us that if David and Elise stay together, each will be denied the most important thing in the world to them - and the world's fate will be affected, as well. Now, that there's what we call stakes.
You'll see the same idea operative in the upcoming Arthur remake (unless they've entirely screwed it up): millionaire boy-man Arthur can have true love... or he can have the millions that keep him happily afloat in toys and booze - he can't have both. This clear and simple dilemma is the high stakes motor that keeps us on the ride.
Conversely, in a recently-bought studio project for which I just did notes, this kind of either/or is what was M.I.A. We had a guy impersonating someone else in order to win back the girl he loves... but the writer had neglected to provide the OMG-factor in what the guy stood to lose, if he failed. So the protagonist's wildly elaborate ploy, albeit comedic, didn't feel justified. Last year's Going the Distance had a similar flaw; Justin Long's character refused to relocate for his long-distance romance with Drew Barrymore's, but the movie never gave us a strong enough reason for why he couldn't move.
So why should we care? With casual sex a cultural norm, and marriage losing the statistical battle in America, whether one's protagonists roll into or out of bed together is hardly a matter of life and death. All the more reason why the contemporary rom-com screenwriter needs to design a better mousetrap for a movie's lovers to trip. Now more than ever, big screen love needs a truly big counterweight to hang in the balance.
I humbly suggest that romantic comedy writers follow The Adjustment Bureau's paradigm. Make both of your protagonists face such a choice: I can have the love of my life... or I can have the thing I've been wanting to have all my life. Put that kind of lose/get at stake, and you're loaded for rom-com bear.
And hey, while you're at it - get Emily Blunt to star in the thing, will you? If I've ever seen a romantic comedy heroine waiting to happen, it's her.