TheNextSabatini, yes, she is young, but, is she hungry? I don't think so. She was setting records long before the current top players were around. Generally, people stay in a profession because they love it or because they don't feel they've accomplished everything they set out to or because it's too lucrative to quit. She's made her money and at the young age of 22 the world is her oyster.
I can't comment on the severity of the injury. But, I personally think she still has the game to win because she kept getting so deep in the tournaments of the last few years. I just don't think she cares enough to study the heavy hitters and tweek her game to get over the hump. For example, she's got great hands, she moves well, and she knows that power players generally like to return power. Therefore, play the net, slice to the lines, and eat up every weak serve/return you see. It's easier said than done, but, she has the skill to pull it off.
Finally, you may notice there's no great clammer for her return from fans, colleagues, or media. I think some of that has to do with a thirst for the next new thing, some of it is people dealing with the reality, and others giving her the respect and space she needs to make her own decision. I think she's making the right move. I also thought Michael Jordan and Magic Johnson should have stayed retired the first time.