The human X chromosome has been (99.3%) sequenced! I know you're as excited as I am. But really, it IS exciting. This is the beginning (well, rational next step) of our understanding of all things gendered! What can we learn by studying the human X chromosome? Well...
1) What makes women and men different? Men have one X and one Y; women have two X chromosomes. The X and Y share very few genes. What do men have that women lack? (Very little.) What do women have twice as much of? (Quite a bit, though we don't use most of the extra.) Knowing all the genes on the X chromosome can help us answer those questions a bit better.
2) Human evolution! Actually, mammalian evolution, too. Apparently, the X and Y chromosomes were interchangable until about 300 million years ago, when they started to diverge. Eventually they became so different that they could no longer match up in cell division.
3) X-linked genetic diseases. There are TONS of diseases caused by genes carried on the X chromosome (hemophilia, colorblindness, Duchenne muscular dystrophy, etc.). By sequencing these genes and the whole chromosome, it's easier to study them. (Believe me, I work in yeast, where the whole itty bitty genome has been sequenced for years, and it's so much easier when you know all the genes are out there just waiting to be characterized!)
4) X chromosome inactivation! This is my favorite. I read/wrote on it for my qualifying exam. The deal is that since women have twice as many X chromosomes as men (and therefore twice as many X-linked genes), we have extra gene expression, which could be bad. So earlyearlyearly in development our cells undergo Lyonization, or X chromosome inactivation, where one of our Xs is chosen randomly and it gets silenced and rolled up into a little ball and shoved in the corner and not used (except when we're making eggs). But it's not so simple as that. The whole process is pretty complicated and not well understood, and that's why it's really cool that the whole chromosome is sequenced, because now we can look better at which genes are silenced and which aren't, and also at the XIST RNA, which does the silencing (and is encoded on the X chromosome).
But don't take my word for it. Nature is allowing open access to the original article and all the commentary. So go check it out! There are some cool pictures too.