For many of us who write part time, time is the biggest enemy we have. Still others its grammar and sintext. After all if you're writing 9th century druids how do you stay within the sintax of the era without having been there.
Research is the key. I spent more hours reading about Ireland and Viking history than I did doing anything else. It took me months to get the information I needed and even then not all of it was correct. There was contradictory information, vague half details, and enough road blocks that more than once I thought about switching time periods. I stuck with it and am happy with the end results.
Of course beyond the syntax of the story there were other challenges. Challenges that I don't really understand because lets face it English is not the easiest language to speak or write. Now you're saying, yeah right. It is so. Nope, I've got friends all over the place that speak and writing in a variety of languages and more than once they've asked me about something and I haven't been able to give them an honest answer - simply because there's three rules for everything we say or do.
Then of course there's the plot. Sticking within the plot would be easy, if you weren't stopping and starting. There's a very good reason that you should have an outline - it'll help you through the dry spells when you're writing. If you just wing it and leave it up, then when you go back, there are plot wholes and characterization problems that may be the deal breaker.
Which brings me to the fact that I love my crit partners and my beta reader. They'll catch things that I would otherwise miss. I can't say thank you often and loud enough.
So those are my pet peeves about writing...what are yours?