What's the difference between the old and newer, more secure URL's for unlisted videos? eg. More entropy?

Any entropy at all. Supposedly the old ones were an encrypted counter, so if that key had gotten loose it would have been bad.

The key did get loose. Thousands of engineers had access to it and it showed up on dark corners of the internet for sale. A bunch of my unlisted videos got more view counts than they ought to have had, so I think abuse of it was pretty widespread.

Probably a bunch of people got promoted for making the new URL and nobody was going to get promoted for maintaining the old one