You're exaggerating the benefits slightly there. A quantum computer can't really perform any tasks a modern supercomputer with tens of thousands of cores can't; it can just potentially do them faster.
You ought to rethink your examples too, since they can't all be done instantaneously. The chess example for instance, you can't calculate how a particular move would turn out, because you need to know the outcome of the moves before it. So you could calculate all the potential moves that turn at once, but you would have to wait until that was done before calculating the next moves, and then wait for that before calculating the next, and so on. So, theoretically, the number of cpu cycles it would take to calculate all the moves would be the maximum number of moves it's possible to make from that point until the end of the game. Which, when you think about it, is in practice actually infinite.
I have a feeling calculating pi can't easily be parallelised either, and cracking a security code would in practice be limited by other factors; since each parallel process would need a different input (a different potential password), you would be limited by the amount of data that could be input into the cpu at once - which is far from infinite. AFAIK, minecraft is a single-threaded program (unless he's changed it).
That said, for tasks which can be parallelised on a massive scale (ie. the kind of tasks which can currently make use of all the tens of thousands of cores available in traditional supercomputers), it can offer a potentially massive speed improvement. Just, as it stands, they're no more useful for most things than traditional supercomputers.
I don't actually know much about quantum computers; but if their only practical difference is unlimited parallel processing capability, then they're actually not as useful as they sound. I don't know if they have any other advantages or disadvantages, though.
