I actually wrote this as a comment in Ken’s blog, but knowing that there’s a better chance of getting more replies if I post it as a blog entry than there is if I post it as a comment, I’ll repeat myself here:
Another issue I had with "online atlases" is trying to understand what constitutes an online atlas. The atlas is very much a product of bookbinding. The binding makes the atlas– otherwise it’s just a collection of maps. Which is what the Rumsey collection felt like to me– a collection of maps. (Seriously, though– I know it’s in the name, but that didn’t occur to me until I was typing this.)
What elements could be included to make an online atlas more than a collection of maps? To "bind" the maps together, even if it is in a nonlinear fashion? What work do atlases do, other than keeping our maps from getting all over the place? And is it worth the work, both conceptual and time in doing the design, to make an "online atlas" proper? Or is a collection of maps enough?