I agree, actually: Kim Clijsters is just about the strongest girl on tour; she hits with big topspin off both wings, can take the ball early, and has an ability to take the flat power shots of her opponents and push them back deep into the court. And with her speed and variety, well... she could (should!) be the best, along with JHH.
The simple difference, of course, is that while JHH becomes more focused and hungry when she gets to the big moments, Kimmy gets VERY tense. You can see it in her ground game... she loses BOTH mph and accuracy, often spraying easy shots well wide or well long. Which is truly a shame because when her game is clicking she has everything going for her. Watch how she utterly destroys her early round victims... it's classic!