I’ve lost the link, but there was a nice list of three things that will discourage a programmer.  I only remember two.

1) It’s too easy.

There’s no thrill to building something you’ve done a couple of times before.  This is an easy way to discourage a programmer.  However, if you incorporate new technology or find different, more efficient ways of doing that same thing, you can make it fun one more time.

2) Nobody will use it.

This is the big one.  If I have feeling that nobody will use what I’m doing, forget about it.  I will have zero motivation.  It’s why I’m having a hard time doing my personal project, Zonbi.  I don’t think it will be used.  There’s too many MMO games out there, so I’m strugling on a reason to make it, and with reasons that would make it stand out.