There are places like Steam Greenlight where you present your game / development process, and people give you their opinion, best wishes, etc. And sometimes they give you their ideas for game features, whether you ask for it or not. Sometimes they send it to you in a PM / tweet: "Hey, you shoud really add this bla-bla."

Whats the deal with this? What if you do accept their idea (or you already wanted to implement it anyway)?

I gather that what you're asking about is "what if the person later comes back and demands royalties for using his idea?" In the case of an idea you'd already been planning to do:
1. Keep the PM or tweet, so as to document the date of receipt, together with dated copies of your GDD that already contains that same idea.
2. Talk to your lawyer about how or whether you should respond.

Or, if the idea is one you had not yet planned to do:
2. Talk to your lawyer about how or whether you should respond.

That is why many game companies prominently state in their TOS or website policies that anything you submit becomes property of the company, AND that unsolicited submissions will not be accepted. The lawyers usually include additional similar clauses.

They help protect against a company working on a game, and someone submitting an unsolicited idea about a very similar game, and then the unsolicited submitter claiming the company stole it.

When game ideas are solicited, the submission agreements tend to have language along the lines that the company might already have similar ideas in development, that they will try to keep the submitted idea isolated, but that the company won't be liable for anything if they make a game similar to the submitted game.

Thanks! Im not in the situation myself when someone came back asking for money or something, but I do like to follow the dev process of small games, dev-logs. Giving and accepting ideas for small features seems kind of natural in the small-indie scene. I guess the legal risk is low, as they dont seem to care.