When a critic says that one of your characters would be better off dead, because they are so boring, I have the impression that this person's interest is not an impartial review or to help you develop your writing skills, but to put you down. Disregard anything that person says.
People are different. For that reason, you cannot create a love interest that everyone will find attractive.
When it comes to physical apperance, one trick I like as a reader and employ as a writer is to describe your love interest as little as possible. If you write that the love interest is tall, dark haired, and muscular, those that prefer normal-sized blond men with a cuddly belly will find him unattractive. But if you write that "he was strikingly handsome", everyone can project their own fantasies onto that blank and fall in love with their own imagination.
That is a strength of writing as opposed to movies: that everyone can imagine the love interest however they like.
Of course, sometimes your story requires specific traits, but keep them as general as possible and only describe as much detail as you must.
With this trick, you can keep the appearance of a character largely unspecified. But when it comes to his or her behavior, you must narrate it. And since attraction is not only to a person's appearance but to their behavior as well, here you cannot but dissapoint some of your readers. If a love interest is a macho, some will find that exciting, while others will be turned off. And the same goes for any kind of behavior.
Therefore the first rule, when you want to write an attractive character, is to write him or her however your protagonist would find it attractive. That way your story is logical and that is half your rent. The other half is simply pleasing yourself.
If you want to write a man who acts in a way that female readers are attracted to, write him so that you are attracted to him. And it doesn't matter if you are a man or a woman, gay or straight. Always – within the confines set by the requirements of your plot and the character of your protagonist – write the love interest so that you feel attracted to how they behave.
In practice, what you do is:
- get in tune with yourself; stop rationalizing and feel who you are and how you are in this world; trust your emotions and your intuition; be yourself
- get in tune with your story and its protagonist; know the setting, what must happen, and who the person is that goes through it; become that person in that world
- observe what goes on and write it down
- rewrite and polish
This, of course, is what you always do when you write. There is no difference between writing the love interest and writing anything else.
If your writing makes you feel what you want your readers to feel, then what you have written is good. It may lack some grammatical fine-tuning or stylistic polish, but the basic form is correct. Ignore what those say who are not your target audience.