I don’t think one can downplay the significance of Mark’s departure because the contribution he made to the development of Photoshop and Lightroom has certainly been enormous and it goes without saying that his presence will be missed at Adobe.
I've heard of Mr. Hamburg before, and know him to be a prime instigator behind a lot of Adobe's success.
As for what Mark is going to do next, it is known that he is now going to Microsoft in Seattle, and that his future work there won’t involve digital imaging, but instead be focussed on the “user experience”. So remember Jeff’s advice and be careful what you wish for!
That I didn't know (unsurprisingly). All I can say is… godspeed Mark, may Redmond treat you well. (though I have my doubts).
UPDATE: I re-read this post the morning after I wrote it.
NOTE-TO-SELF: Proofread at least twice before posting when you're tired.
Real UPDATE: Further news from photoshop news. Apparently, Mr. Hamburg is going to be working on revolutionary new OS interaction methods. Seems like this is the hot field for programming geniuses to be in nowadays. And it makes sense. The last/only great evolution we've seen was the mouse/desktop analogy paradigm. I'm not sure if Microsoft will be the company to come up a new way of interacting with a computer, but here's to Mark's efforts!