Great question.

For our team, we say yes to whatever solves the biggest pain, for most of our users.

We discover this by reaching out to customers, and doing a lot of listening. We're trying to answer two questions:

1) What is your biggest pain in your daily work?

2) What is your biggest pain in our software?

#1 usually leads to new features.

#2 usually leads to revisions of what we have already.

it usually helps to turn the question around - "what if we don't do this?"

end of the world? end of the customer? etc.

same triage as in medicine. once customers get silent, it gets dangerous. as long as they are screaming, they are using your stuff and want more.

When it answers a need that a sizeable percentage of your users have.

When it makes things easier for a sizeable percentage of your users.

When it can be done at a reasonable cost.

When someone with significant clout specifically hires you to. - In enterprise software it sometimes makes sense to build things to order for a specific client, even if just as one-off project.

When it saves you money behind the scenes.

And, to a certain extent, when you can rapidly prototype it and don't know what will happen - you can sometimes spin it off into a separate product if you're large enough.

Note that need and easier are not things that the users necessarily know themselves.

Sometimes your roadmap is for sale, it happens (http://www.bothsidesofthetable.com/2013/04/15/how-to-sell-yo...). But you know when to say yes by having: 1) A vision of what your product is going to become 2) Understanding of your customers and your market

Only when these things intersect do you say yes.