My goodness, I haven't ranted in a while. Let me rectify this oversight! :)
I have seen the following question asked in a couple of different places. It goes something like this: "I'm working on a novel, and (a) it's in a genre everybody knows is overcrowded but I love this idea, or (b) I just found out that two books very much like it are coming out next year. Should I write it anyway, or should I shelve this book even though I love it?"
It probably comes as no surprise that the usual, very passionate, advice is "Write what you love!"
And I couldn't agree more! What troubles me, though, is the implied corollary: "If you love it, you must write it. If you love it, that's justification enough." I dunno. I wonder if it wouldn't be more accurate to say "Love what you write!" As in, yes, I will commit to a project only if I love it. Only if I think it's "mine" to write. Only if it really taps into my heart. But does it follow that if I feel this way about a project, I must write it, even if there are reasons to consider not doing so?
No. In the first place, most writers are already in a position where they will never write all the ideas they love: There are too many for one lifetime. So while love is necessary, it's not enough to assure that any particular idea moves from the "great ideas" file to the "finished manuscripts" file. To wax mathematical for a moment, love for an idea or story is a necessary but not sufficient condition.
Unless you know you are writing a practice novel, I don't see how "Never mind the market and just go for it!" is the best advice. In days of yore, after selling a MG series written in third person, I started a mystery series in first person. The MC decided she was going to speak for herself, thank you very much. "No," my editor on the first series said. "We don't do first person." Period. Even if I loved the project, or had done it well, it still wasn't marketable to that house. Fortunately, I was able to sell it elsewhere, but that doesn't change the fact that love doesn't conquer all. It's a necessary, but not sufficient, condition.
More recently, I caught "the last train out of the station" with a certain genre. I wasn't thinking about what was popular or unpopular in the market. I'd just found a great idea and dove into passionate work on it. I was about 3/4 done with the book when I started hearing that this genre was saturated. What's more, I found out that two other books on my specific subject within that genre were scheduled to come out within the following year to year-and-a-half. No, those agents were not going to take on what was essentially a competing book, nor were those publishers. My book's competition was too direct, and there wasn't room for another, especially since it was a latecomer. My love for the project could not change that.
Many would encourage the writer to stick with a project by saying, "Your book will be completely different from the other book because you are a different writer!" Well, maybe it will and maybe it won't. No one can blithely promise you that your book will be completely different. There can be uncanny resemblances between your work and somebody else's. And some topics in themselves are too specific to stand much competition.
I think "Write what you love/write what will sell" is a false dichotomy. I insist that the twain shall meet. I want to write salable work, and I'm willing to tweak both the love and the market aspects so that I'm writing something I love and that isn't market-handicapped. It just seems to me that since I love a lot of different books, there's always a new idea to love, and in the end I can have my story and (hopefully) sell it, too.