When writing, we frequently want the readers to relate to our characters. We want them to see something in our main character that rings true. We might want to give little anecdotes that make them stop and think things like, "Oh my God, I nearly peed my pants when that happened to me!" It makes the character seem more real and thus more realistic. But at what cost?
I am currently working on a story where the main character is a vampire. This makes the decision easy and the application a little harder. I can't make a character that the reader can completely relate to because none of my readers (I would hope) are vampires. But I can make them relatable in other ways. After all, she wasn't always a vampire. She has a past that can be related to. Even some of her actions and mannerisms can connect with the reader's day to day life.
But while that connection to the reader is important, what makes him or her different is equally important. First, you can't make a character that will relate to every reader, even if you have a very limited demographic. Second, it's boring. Readers (at least I) like seeing a person's quirks. It gives the character depth. Like not liking coffee or Christmas or sunbathing or liking the number 13.