Best post I've seen on the topic (from Ev Williams) is here:
I don’t know if Evan put it first on purpose, but I tend to think that tractability is the most important factor by an order of magnitude. Why? Because there is a pretty good chance that what you think you’re going to build and what you go to market with are radically different. Take a few minutes and ponder this outstanding quote from Fred Wilson’s “Why Early Stage Investments Fail” post:
“…Of the 26 companies that I consider realized or effectively realized in my personal track record, 17 of them made complete transformations or partial transformations of their businesses between the time we invested and the time we sold. That means there a 2/3 chance you’ll have to significantly reinvent your business between the time you take a venture capital investment and when you exit your business.”
Another great quote:
“My friend Dick Costolo, co-founder of FeedBurner, describes a startup as the process of going down lots of dark alleys only to find that they are dead ends. Dick describes the art of a successful deal as figuring out they are dead ends quickly and trying another and another until you find the one paved with gold.”
Given that you have a 2/3 chance of having to reinvent your business (or, as Costolo would put it– you have a 2/3 chance of your first dark alley being a dead end), what could possibly be more important that tractability?
Of course, there's the passion argument. If there's something you REALLY WANT TO EXIST IN THE WORLD (which is different from something that you think you'd enjoy building), then you might want to chase that, tractability be damned.