I doubt there would be any unannounced features. Snow Leopard didn't really have any. Any major things would require developers to know about it to be able to upgrade their apps, so Apple could not just change such major things at the last minute. Devs need to be informed in advance.
I think that Lion has loads of features that make it much better than Snow Leopard, some are more fundamental and more advertised changes, such as Mission Control, Resume, Auto Save, Versions, and all that. However, I think that the little things (see "All the little things" thread) are way more interesting. Things like being able to enlarge windows proportionally while pressing Shift, grouping selected files into a folder with a keystroke, new and better ways to organize files, disappearing scrollbars, reverse scrolling, year view in iCal, and loads more stuff make the overall Lion experience much better than Snow Leopard.
I also think that the new multi touch stuff in Lion will really change the way you use your computer. Swiping through spaces vs. using the arrow keys might make you use spaces in a very different way. Maybe you'll end up moving windows around a lot less or a lot more, either way, it's a big change.
It's true that Leopard had Time Machine, Stacks and Spaces, three fundamental and big features that really make it a worthy upgrade, in addition to lots of smaller things including a new dock. But I don't think it's possible to come up with such amounts of new and useful things every few years.
That being said, I have not tried Lion and I think it is quite possible that Lion - with its small-to-medium-sized features - will bring more change to your life inside the computer than Leopard did.
