A very subjective question... I'm assuming the term game equals fun here? Otherwise I would say life is very much a game, a very hard and sometimes rewarding one. Level infinity to be reached... but the rules are never fair, consistent and you're playing the biggest MMO there is! We're all on the same team, but trying to do different things with different people… Lets just say the developers really need to patch this one to death, or it's a huge social experimenter they want to be chaos. Then is it just them playing a game we're simulating?
Many say life is an illusion, something that exists for a time before returning to the universe as base atoms if not smaller constituents. So in this sense, yes reality is an illusion of the present and can be taken less seriously (as a game).
But of course because we interpret and interact with everything through our own senses and experience, we are often restricted to what we are taught/perceive and it seems very far from a game to most...
Some people have more fun taking the world as a game, but that requires a lot of fearless drive in my opinion. The world is generally not run by men of equality and fun, there are rules, fears, oppression, restrictions and bias everywhere. You would really need to reject all these factors, loosing yourself to moral or immoral purpose to play it like a game. Life seems too serious otherwise :P
Yet some are blessed with the ability to perceive and reject the constructs we hold dear, they are truly wonderful and bizarre at the same time (but so is the nature of the universe, the biggest game alive). I am very jealous of such people, they can have more fun, play more games and achieve more change, if they are not perceived as a threat by powerful parties in the process.
I do believe you need to play life as a game to the best of your ability, you can shape and change it's opressive nature to bring out the brighter more hopefuls sides and even reject popular society games all together (self sustainable and alternative movements, etc...). But in the end it comes down to what you believe you can do, the amount of power or what level you think you are, the alliances you make or don't, and how much punishment you enjoy in a game :)
Life can always be called a game I believe, but who is playing it, what the rules really are, is it any fun and to what end??? It changes so much, who knows?