Can someone convince me that this is more impressive than it looks? Isn't it just a case of building a big-enough database with associative keyword nets, which was already well underway in the eighties and turned out to be a dead end?

Recall why these seemed to be a dead end. Two major reasons were (as I understand it) that the databases had to be massive to be useful and that they didn't have easy ways of adding facts and relations without human intervention. Watson helps show directly that the first is no longer as big a deal, and will be less of a problem as computers improve. The second is also less of a problem now since Watson can take large datadumps and then develop associations more or less itself (at least that's how it was explained to me.).