That's not really true. Bethesda released The Shivering Isles expansion in response to the crappy micro-DLC they released before. The Fallout 3 DLC was also relatively substantial and some of it was actually pretty good (Broken Steel, The Pitt and Point Lookout specifically). Hearthfire is a significant departure from the trend set by Fallout 3.
I wasn't arguing that Oblivion never had any good expansions, of course not. I was arguing that after releasing a magnificently successful game, they shouldn't immediately follow it up with lackluster content. This unfortunately just makes people quickly lose interest in DLC for the game, and move on. I think they should have waited and let their first post-release content be something far more significant.