Sorry--my post was a bit of a general musing.
A bit of what Greg was saying.
One of the largest (and most intangible) changes is the gaining of perspective. Greg mentioned how one appreciates 3rd year of med school differently after years of doing experiments--and I am greatly looking forward to this aspect.
You can really tell, even in graduate school, the difference between a new medical/grad student and more senior students. In both pathways, wisdom (or some semblance of it) is gained through experience. I know that I feel quite a bit different now, having almost completed my PhD, and probably manifest this in more mature behavior, more careful thought and consideration of problems. I know better what it takes to collect, carefully evaluate and analyze data, publish papers, get grants, defend my work, put together a complete story, network among peers, present findings, answer questions in a careful, thoughtful way, etc. Much of this gets summed up in "learning how to think like a scientist." But of course there is much more to this process, which often receives little attention, and therefore little appreciation.
There is also a feeling of accomplishment in having completed a long-term, large project that has contributed in some way to our understanding of the world, and perhaps down the road to disease-related applications.
The bumps and bruises along the way also go a long way in building emotional and intellectual resiliency ("whatever doesn't kill you makes you stronger" comes to mind). There is a whole interesting line of research on how experiencing stress in a controlled setting enhances an animal's mechanisms for dealing with future uncontrolled stress. I think this most definitely applies here.
Finally, going through this process has given me a new appreciation for listening to and understanding the experiences of others through careful observation and thought, qualities which I hope will be of value on the wards, during residency and in the future as a physician-scientist.