For obvious reasons, we'd all rather have a medium stack than a short stack. But there is one advantage to being short-stacked: your decisions are easy. With a medium stack, almost every decision you make is complicated and almost every move is awkward. A medium stack is defined as when you have about 30-40 big blinds, and learning how to play it is absolutely vital to tournament success. In most tournaments, you'll have a stack that size from about the time half of the field has been eliminated all the way to the final table.
This means you'll potentially spend up to half of your tournament in a position where every pot you play is pivotal and every decision is tricky. You have far too many chips just to open-shove, like you would with a smaller stack, and if you open for a standard raise, that's already close to 10 percent of your stack that you're putting at risk. So you can't play too loose.