I think most writers try to compose interesting characters, and put a lot of work into creating an appealing cast for their story. Reading a novel with boring characters is like being locked in a room filled with everyone's least favorite aunt, the sort who wants to pinch your cheek, tsk over your clothes/weight/love life, and talk about their bunion or bowel problems without stopping for breath.
(Aunt Frances, I'm not talking about you. I promise.)
One thing I do notice about characters in novels is when the cast is unbalanced. Raise your hand if you've read a story populated by the following:
All of a kind: the characters have interchangeable or sound-alike names.
Gang Bangs: the cast consists of one female and twenty males, or one male and twenty females.
Generation gappers: the entire cast is in their twenties, or thirties, or forties, etc.
Monochromatics: all of the characters are white, or black, or Hispanic, or [insert ethnic group].
Threesomes: three siblings, three friends, or three of a kind.
Tokens: one black character in an all-white cast, one white character in an all-black cast, one gay character in an all-hetero cast, etc.
West Side Stories: all the characters are split between two opposing groups, no in-betweens.
Now before anyone starts sending me hate mail, I'm not knocking every story with these type of casts. Some of them have become a tradition in genre fiction, and I don't think they're unbalanced if they serve the story. Threesomes, for example, are the time-honored foundations for romance trilogies. If you're writing a book titled "The Last Woman on Earth" you have no choice but to use a gang bang cast. Casts for epic fantasies are prone to become West Side Stories because of the good vs. evil conflicts involved.
That said, if you're tired of the same-old same-old, and want more diversity and originality in your novels, it's a good idea to balance out your cast before you start writing the book. First, make a list of all the characters in the novel by name in alphabetical order. Then:
Ages: Note the characters' ages next to their names, and compare them. If everyone in your novel is 29, and it's not titled "Logan's Run" you might want to shift around some birthdays.
Gender: Highlight the names by sex: pink for girls, blue for boys. If you end up with a huge chunk of blue or pink, you might consider switching some of the characters' genders.
Group Dynamics: mark the members of any numbered group like a threesome among your cast. Are they individuals or blurred clones of each other? Work on how you can give them more distinct personalities so they're not so dependent on their group relationship.
Loyalties: Note whose side your characters are on. See if you have a few characters who are neutral or disagree with both sides. If you don't, think about changing around some loyalties.
Naming: Look at the characters' names to see if any sound alike or start with the same letter; readers often confuse these characters with each other. Think about changing the ones that are too close.
Race: Put a letter indicating race/ethnicity next to your characters' names. If your cast is all one race or ethnicity, and you can't logically justify it, it's monochromatic. Also, use this step to see if you've cast a token character. Unless there is a valid story reason for a monochromatic cast, consider putting a bit more racial diversity in your story.
Balancing your cast through diversification makes your characters seem more alive and natural. They become people who might exist in reality. Since that's the goal of most writers, I think it's worth the effort.