I'd say it becomes advertising when you're directly profiting from said coverage. Even then, I don't necessarily have a problem with it beyond any lack of disclosure. TB for example, did sponsored content for Planetside 2, but he was completely open about the origins of the content. It was still good content, in the same way that Sessler's recent Evil Within videos were good sponsored content.
There are definitely situations that could be perceived as fuzzy in my area of work. USgamer's budget isn't huge and Ubisoft wondered if we wanted to attend a preview event. It wasn't in San Fran or New York, those we can cover on our own. So, Ubisoft offered to cover travel, no contract or stipulations. If there had been any stipulations on coverage, we would've just declined. By the by, most of your larger sites - Polygon, Kotaku, IGN - pay for their own travel. We do for larger events - PAX, GDC, E3 - but other single events are outside of what we can afford.
Where do people fall on that? I'm sure many would see it as a free trip. I flew out to Vegas on night, got to my hotel, ate, slept, woke up, played AC Unity for five hours, got back to my hotel, ate, and then spent the next 12 hours in the odd hell that is redeye airport travel. At the end of it, I wrote a long-ass preview and did one video on the game. How you feel about that is something for you as a reader to decide. My important thing is disclosing that Ubisoft paid for my travel to the event and being honest about my feelings on the game. Again, it's all about honesty and transparency (within limits).