Eduardo was a Distinguished Engineer at Sun and as such, he worked with Sun's top engineers. He was also a senior technical member of the team that created JavaEE.
He's not just a productivity dynamo, but he's just a great coder. This was exemplified when I idly mentioned in the IRC channel "it would be pretty great if there was a way to unit test my plugin." About 10 minutes later, Kohsuke responds with "well, there is. There's a test harness I wrote." The test harness is a thing of beauty, spawning a headless, up-to-date version of Hudson that allows all sorts of querying about its operation. I'd never worked with something quite as amazing.
Kohsuke == Hudson. Hudson without Kohsuke is essentially just an historial artifact. Oracle stands to lose much more from a potential fork than they realize.
EDIT: The man also has the patience of a saint, which cannot be overstated when it comes to support.
A good example would be Akuma: his embeddable UNIX daemonization library that he wrote for hudson to use: