Here's the old download manager window.
|Firefox download manager in Firefox 19|
And here's the new download manager UI.
|Source: Firefox Help|
Things are much more streamlined now.
Of course, that's not all. We're also getting per-window private browsing which Chrome has had since... forever.
Finally, the usual performance improvements, security fixes, and HTML5 improvements are also rolled in. Let's not forget plugins no longer bring down the whole house when they die.
Read more in the release notes.