Edit: The more I use it, the more I like this thing. It's hard to put my finger on it, but the small size of the old UI definitely made it feel a bit like a second-class, constrained tool compared to the desktop.
Interestingly, my impression is exactly the opposite. The oversized new UI feels like a constrained toy to me, compared to my usual file manager(s).
On my desktop I can make icons 16px and easily manage hundreds of files and folders without even scrolling, or switch to a thumbnail view if I need to see larger pictures. The Dropbox web UI, on the other hand, has only one view option that seems ideal for neither documents nor pictures, and it doesn't make any use of horizontal space beyond the hard-coded 960 pixels. So it still feels too much like a website and not enough like a file manager application.
But that's not necessarily a bad thing. Large icons and bare-bones interfaces might make it easier for people to perform simple tasks on a small number of items. Besides, not everyone is looking for Finder/Nautilus/Windows Explorer when they open Dropbox on the web. In fact, I guess most people use one of those desktop apps to manage their Dropbox most of the time anyway.
Layout:
[ ] Old School
[X] New Hotness
(I like the new design if you can't tell)
Having said all that, I didn't like DB's old design, and it was never completely intuitive for me to figure out how to leave a shared folder that was hoggin up my space...