So...All this time you're saying that passion is not a great thing to have for a game designer?

I don't think anyone's saying that. You just have to be objective, too--or, if you're having trouble with that, playtest the game in its earliest stages to see how people react to it. I'm working on a comedic adventure game right now, and if things that seemed hilarious in my head didn't warrant so much as a polite chuckle during playtesting, then I cut then. I did that because I was passionate about the game and wanted it to be the best it could be, despite my own limitations.

you should be passionate and everything. but you have to know, and always consider, that you can be wrong on something. you're not perfect, sometimes someone else is right. and you have to accept that not everything you invented is pure gold and pure genius.

if you can't let go of ideas that failed/will fail, you're not good at your passion. because your main goal and passion is and should be making a good product. not a product with a feature you've invented, even if that feature is completely bollocks.

so put your passion into a higher level: the actual product (be it a game, a webpage you're designing, what ever).

an idea getting dismissed doesn't mean the idea itself was bad. it means it doesn't fit right now, right here. focus on the project, not your idea.