Personally, I've never really thought about my method towards buying games... But I do have my own technique I guess.
The gameplay trailer itself is the first major influence. Usually, that's the make or break moment for me and my interest (even though trailer gameplay can sometimes be false).
Second major influence would probably be my friend's reviews. Usually, I'm never the first person to buy a new release (I'm not that cool...). However, it gives me a chance to hear the reviews from my friends, as well as online gaming communities on the overall gameplay itself. Without giving too much away (spoiler) of the storyline.
Third would probably be the price. I use to fork out a lot of cash for new gaming titles. I'm not like this nowadays though. Despite this, the overall price of the game does play a key third factor in my overall interest. Also, like
@SashaS said, Steam games do offer refunds which give you somewhat of a comfort in making your gaming decisions. Though the worst idea for me is having to return a game I just bought. Physically or digitally...