My biggest hang up as a writer is naming my characters. Nothing stops me faster or longer than introducing a new character. Suddenly it becomes paramount to find the perfect name. I think about the character, what fits the image in my head, I research names on baby naming sites. I have spent days looking for the perfect name for even minor characters. My last manuscript still doesn't have a name for, guess who, my main protagonist no less. But, when I hit on the one that feels right it sticks. In the manuscript I'm currently writing, my main character had his name long before I wrote the first sentence.
In most cases, I don't feel comfortable continuing with a story until whatever character I'm working with has a name. I had a name for my main character in the first manuscript and three-quarters of the way through the book realized it was annoying to read. That's a major problem. The last thing you want to do is turn off your readers with a name. That goes for naming just about anything I think - a character, a place, the book itself. I'll bet you've read a book that irritated you because a name used often enough tripped your tongue every time you read it. Even an astounding book will suffer that way.
I've experienced writer's block for plot only once or twice. This may or may not be a positive sign, I prefer to look at it as positive. I usually know where I'm headed. However, naming things just waves a red flag into my brain and stops the train dead.
I've been thinking about this problem and wishing I could build a repository of pre-thought out characters with full backgrounds and names. The problem with an approach like that is that the character has to fit the story and incidentally the world you're describing. It might work to modify a character into the story but that seems so forced. Characters have a habit of popping out of nowhere and demanding to be given their spotlight. I prefer to allow this freedom, it perpetuates creativity.