The main thing for me is gameplay. I can deal with not so great graphics or audio or clunky controls. If the gameplay isn't forgiving or easy to work with, I'm not going to waste my time on it. I think we've all dealt with games like this. I think it's probably why I'm most partial to retro type games like platformers and beat em ups.
Lets not forget about advertising though. I suppose to some people this will be all they see before playing a game. I guess what they see depends on what is shown in the advertisement. Some people like to rent first. I think this is a less important factor as far as deciding if one wants to buy a game or not than it used to be as rentals are becoming more and more scarce with the advent of downloadable demos and the like.
What is it that gets you interested in a particular game? What's the most important thing to you? Gameplay, plot, graphics, immersion, etc.?
This time I present a theme from one of the games I've spent the most time with, Diablo 2. It's called 'Rogue'.