I've always loved writing in the first person because it forces me to get inside my character's heart, head and soul and to explore and delve into her/him and then become her/him.
But to write convincingly in the first person, you need to uncover and discover that character's voice -- come up with a voice that is fresh, distinctive and completely convincing.
Here are some common pitfalls I've noticed in books written in the first person:
-- having a generic voice. This might work in a third person narrative (well, only maybe, because let's face it, generic is blah, no matter what point of view you chose to write in) but it is particularly grating in a first person narrative. If it doesn't work and the voice is unconvincing, the book will fail to engage the reader even if the story is exciting.
-- tied to the generic voice is the lack of anything distinctive to make the character's voice singular. If you can't tell who is speaking without saying so, perhaps the voice isn't distinctive enough. And perhaps that points to a deeper problem -- maybe your character isn't distinctive enough. Maybe you don't know enough about your character to write convincingly from his/her point of view.
-- in writing for children, having a voice that is too old. Adults can often have trouble connecting with the child's inner vocabulary and intensity, and the rhythms of speech and thought.
-- using vocabulary that is too old for a child narrator.
-- using expressions that are too old for a child narrator. (Warning here: yes, kids today say "like" almost incessantly, but if you use it in a story the way a kid might in real life, it'll trip the reader and mask the story. The trick is to use it sufficiently to make the voice sound like a kid's voice, but not enough to annoy the reader.)
-- getting the rhythm and pacing wrong when writing from a child's point of view
-- getting the rhythm and pacing wrong for a first person narrative. A first person narrative has to sound like a person telling the story (aside: you can use a first person point of view to have the narrator tell someone else's story, not their own) and as such has to reflect the rhythms of that particular person's voice.
Sometimes it takes many rounds of edits to get that voice right. In the early drafts of my first children's novel, A FRIEND LIKE ZILLA I found myself writing in a voice that was distant -- it sounded like an adult looking back and remembering. I had a superb editor, Charis Wahl, who pointed this out. It took many re-writes to get that voice right, to make the voice of the main character, Nobby, ring true, and seem convincing and particular to her.
Some books are like that. Part of finding Nobby's voice and making it convincing, was getting to know Nobby and making her convincing.
And then there are books where the voice just comes to you. When that happens it's a gift. It happened with my picture book A Screaming Kind of Day. Scully was real to me from the get go. I had her voice clear in my head from the get go.
But regardless of whether the voice of your character is clear from the start or not, it's imperative, if writing in the first person, to make it convincing and unique and true to the character.