Thinking about it, the factor that carries the greatest weight in my perception of a contemporary romance (assuming that writing quality is acceptable) is the love interest. Interestingly, the plot, the other minor characters and even the protagonist seem not to play as great a role as the love interest. For example, let me examine the two Kasie West's contemporary romances to illustrate.
On the Fence vs. The Distance Between Us
The love interest in On the Fence is Braiden, the sporty, funny childhood friend/boy next door.
The love interest in The Distance Between Us is Xander, the wonderfully nice rich boy who seems to be out of reach for the protagonist.
Which book did I prefer? The Distance Between Us.
Looking at the plot of both books, they're actually kind of similar. There's the story arc going from initial character development, then realisation the protagonist loves the love interest, then some sort of adverse event(s), then the HEA. And obviously, being by the same author, the writing is of a similarly high quality.
Why then, was it that I preferred The Distance Between Us? The reason is definitely Xander. He's just so nice. I mean, sure, Braiden is too, but man, Xander is just in another league of nice haha. (It may not surprise you that I prefer the nice guys over the bad boys lol)
Anna and the French Kiss- swoony boy
Lola and the Boy Next Door- book boyfriend
Easy- swoony boy
My Life Next Door- book boyfriend
Speechless- book boyfriend
The Sea of Tranquility- swoony boy
Slammed- swoony boy
Fangirl- swoony boy
You gotta admit, that's a pretty impressive lineup of swooniness. I have two categories in which I sometimes place love interests- "book boyfriend" and "swoony boy", which are pretty self explanatory, so I'll move on haha. You can see that most of my favourites come under one of these categories. I should probably add, it's actually rather rare for characters to be added to these categories- I have only 11 in book boyfriend and 27 in swoony boy.
So clearly, the love interest plays a rather large role in my perception of a book, but what about you guys? How big a role does the love interest play in how much you enjoy a contemporary romance novel?