Your biggest challenge will be to get the appropriate wilderness permit. If you are going solo it should be realatively easy to get one. Regardless, if you intend to go southboand (by far the better way to hike it), you'll need to get the permit at the Wilderness Center in Yosemite Valley.
There was also a very substantial tree blow-down this last winter that has affected the trail, although much of that has been cleared (and even more will by the end of July).
It is a GREAT hike and I highly recommend it. Because of the need for a permit, the fact that it rarely brushes up against civilization, and for some other reasons the most critical element to a successful hike is a good plan. For that, I recommend you begin at JMTBook.com.
Good hiking, Ray
Author, Planning Your Thru-Hike of the John Muir Trail
Follow me at: www.twitter.com/JMTBook