A Game Designer starts with the vision of a game, or a feature for the game, and sets out to refine the idea until it achieves all the goals set out for it. Sometimes that journey results in scrapping the idea entirely because it didn't work out the way you hoped. In this case, you at least worked through the problem and fully understand why the end result came to be. A buddy of mine calls this "Killing Your Babies", not getting so attached to that idea and wasting time when it's not working.
But, sometimes this journey has the Game Designer molding and refining the idea until it becomes something special that fits perfectly how you intended it to! You start creating specs and getting the information out to everyone...ready for implementation to proceed. Then you get a kick in the nuts because some factor completely outside of your control says the feature needs changes or cut all together. These types of requests can come from many places: your discipline lead, Design Director, Producer or sometimes even from upper management.
The question is, how will you deal with this situation?
Talk Out The Idea
There will be the temptation to be forceful or even angry during this kind of conversation, but just realize that the person (probably) has a reason in their head that justifies the decision and you need them to see it your way. Try having a really good attitude about things, understand their concerns, and convince them why your plan is the right direction for the game.
Own The Idea
This isn't an easy thing to do sometimes, but keep in mind that as a Designer you are the beacon holder for the fun of your game and the rest of the team count on you. If you are not, at least seemingly, fully behind the new direction for the game then no one else will be compelled to put in the kind of work necessary to make it the best it can be. I'm not suggesting you fool others here, just saying that you need to get on board and make sure others see that you are.
Justify The Idea
The Design Brief is also a fantastic tool for relaying the vision and getting the entire team in sync about what you are trying to create. Discussions can get passionate at times and everyone has their own preferences which they push for. The Design Brief really helps minimize heated conversations and make sure that you are all narrowing toward the same goal.
The most important thing to keep in mind is that there will always be situations out of your control, but you can control how you deal with it and the message you send to your team. In the end, you may feel like you are compromising with an inferior feature but you have to put everything you have into making that work as well as it can for the game. The entire team is going to be looking to you, do you want them to be bummed they have to work on something or focus on how great the new thing can be if everyone works together?