Talking about a game in a Steam review, broadcast, curation, Greenlight upvote, or even forum post? If it's a compensated promotion of any sort, all Steam users — yourself included — are now required to publicly disclose that info.
"If you use Steam services (e.g. the Steam Curators' Lists or the Steam Broadcasting service) to promote or endorse a product, service or event in return for any kind of consideration from a third party (including non-monetary rewards such as free games), you must clearly indicate the source of such consideration to your audience."
The bit about non-monetary rewards seems to cover things like free games offered in exchange for Steam Greenlight endorsements — a practice Valve isn't fond of, but they can only do so much to stop.
This strikes me as a necessary step given that Valve is fully on board with the idea of a Steam train whose engines are powered by users. If nobody else is holding these people accountable, that responsibility falls to Valve. Problem is, it will take a lot of time and energy to fully enforce this policy, and I doubt Valve has the manpower for that. In an era of sometimes questionable video game promotion deals, it remains to be seen if a fancier version of a sign that reads, "Hey, don't do the thing... OR ELSE" will keep people honest.
Moreover, language like "you must clearly indicate the source of such consideration to your audience" leaves an unfortunately large grey area. What does "clearly" mean in this case? As YouTube has shown us, people can interpret that kind of requirement in many different ways. I feel like standardised disclosure — like a check box that adds a badge with a brief explanation to your post — would be better. People would know where and what to look for, at least.
And again, as exemplified by the aforementioned Greenlight issue, some of Steam's systems encourage game makers to hand out what essentially amount to promotional rewards on a large scale, whether Valve intended them to or not. This isn't, in other words, just an issue that concerns reviewers and YouTubers. I hope Valve can figure out a way to make this rule stick, but you'll forgive me for being more than a little sceptical, given the sheer amount of individual policing it will require.