Regina McCombs has a basic but thoughtful post on Poynter Online:
Meaning in Motion: Ken Burns and His 'Effect'
Zooms, tilts and pans can enhance impact or induce illness. The master of movement on still photos offers insight on managing multimedia.
Update: see also interviews and more in The Ken Burns Effect — and beyond. Here's some history from Donald S. Berman on FinalCutPro-L (10-5-2011),
'Ken Burns "developed" the effect named after him with Ed Joyce and Ed Searle of the Frame Shop, located at that time in Watertown, MA. Burns used to hang around our edit rooms learning how to handle large quantities of documentary footage. Joyce had spun off from Education Development Center when its film operation closed down. Joyce and Searle had set up an Oxberry and also had developed motion graphics techniques with computer controlled cameras. ... Ken got the credit and, eventually, the Frame Shop went bust and there are only a very few of us around to tell the tale.'
Here's some more from others in that thread:
Before The Civil War, who thought a crazy-long documentary composed of talking heads and old photographs would draw and hold such a large audience?
He didn't invent moves on stills but as far as I know, until Burns, no one made a 9 episode, 10 hour series that was so dependent on them nor was anyone willing to linger on a specific still for the length of time Burns and his creative team employed.
I admire that he trusted that if viewers were given the right context and the opportunity to actually look at photographs and documents (without white flashes, snap zooms, swish pans, elaborate photoshopping/AE work, or even their replacement with "reenactments") the stills would sustain the viewers' interest. In my experience, most of the time, stills are seen as the thing you stick in when you don't have footage to avoid having nothing on the screen or a jumpcut.