The format of the new URLs (addresses) have been designed for brevity, cleanliness, internationalization (as little English as possible), future directions, and correctness.
The old addresses still either work as is or redirect to the new ones. There are lots of places on the site linking to old-style URLs still, but those will be fixed over time.
I'm sure there will be some objections to the new format (as happens with all changes), but we did discuss the changes quite a bit and made a bunch of changes as a result of the discussion. The relevant posts/threads are:
We've been testing this quite a bit, but little problems are sure to pop up. We'll be around all day (as we almost always are) to fix any problems with the change.
The next two birthday announcements will come in 12 hours or so, so don't stay glued to your monitors. :-)