Here's my take on them:
How do you prevent inflation in a virtual economy?
Needs improvement. Why? Because of nebulous phrasing like, "your typical MMORPG". What "typical MMORPG" are you talking about? Who decides what a "typical MMORPG" is? Is that some code-language for "WoW-like"?
The question as stated is too broad to really deal with. It needs to be revised to adequately explain the specific mechanics at play, which ones are flexible and which aren't, etc. Otherwise, it's all left up to the person answering it to decide what it really is all about.
Should collisions affect the car engine in a racing game?
Also a bad question. Why? Because it's purely opinion based. Here's what I mean.
If I'm making Mario Kart, collisions affecting car performance is a really bad thing. However, if I'm making Gran Turismo, collisions affecting performance is a really good thing.
But your question doesn't state what kind of racing game. It makes the incredibly ridiculous assumption that there is one correct answer for every possible racing game. There are a multitude of different styles of racing game. What is appropriate for your game depends entirely on what you're trying to do.
In short, it's not specific enough. What you're asking is for people to say which gameplay they like better. Maybe they'll explain why, and maybe they won't. But ultimately, you're not going to convince anyone to change their mind on their preferences, and in the end, the answers will help no one.
If I'm looking to make a Mario Kart-style racing game, you're not going to find an argument that will convince me that it's a good idea to include collision damage. And if I'm making Gran Turismo, you won't convince me to take collision damage out of the game. Thus, nobody comes away with any actual wisdom or knowledge, and it was all just one big waste of everyone's time.
Game design questions should not be so theoretical. They should be about practical design problems: "I have a game that works like this. Players of that game seem to be doing X. I want them to be doing Y. How do I make that happen?"
Game design is about problem solving. How do you make each element of the game fun and interesting in the way you want it to be. GameDev.SE is not about high-level conceptual questions. Design questions should be about the nitty-gritty of actually designing a game from front to back.
Of course, the problem is that most hobbyist game developers will never get that far. And since they seem to be the majority on the site, we get a bunch of high-level garbage questions that never get enough close votes to die.