Personally, I think computing power has hit the point where Java desktop apps and applets aren't that big a deal. I've got one running on my desktop right now. It runs for day's on end without being a significant burden to the rest of the system.
Many still write code without thinking one second about performance.
Java isn't C, but if you code thinking about performance it is possible to write quite usable applications for many use cases.
And even for that "It runs for day's on end without being a significant burden to the rest of the system" is hardly something to write home about.
Just as pretty as Keynote? I beg to differ. NetBeans (and Eclipse, IDEA) always hit the "uncanny valley" for me (and Eclipse is even using Cocoa!). Not looking right at all for the platform, and with BS divergent behaviour on lots of UI controls. On top of it, all three are quite laggy (gotten a little better with huge memory these days and SSDs, but still, if you need 4GB and solid state to get something to run fast, well...).
In no way I would say NetBeans provides a better experience on a Mac than Keynote.
Having said that, not all apps have to be excellent 100% in terms of usability and beautiful-UI. Once I step out from the HN-bubble a bit I'm starting to see people using ugly but usable and solved business problems type of app written in anything from Tcl/TK, Java, Qt, Pascal, etc.