I wasn't reading OB before LW existed, but if you look there now, it's immediately apparent that the topics represented on the front page are much, much, much more interesting to the average casual reader than the ones on LW's front page. I wouldn't be surprised if the commenters tended to be less invested and less focused as a result.
(EDIT: I shouldn't say the "average casual reader," since that must mean something different to everyone. I clarified what I meant below in response to katydee; I think OB appeals to a large audience of interested laymen who like accessible, smart writing on a variety of topics, but who aren't very interested in a lot of LW's denser and more academic discussion.)
I suppose I'm not the average casual reader, but here's my comparison--
Less Wrong front page:
-Occam efficiency/rationality games-- low interest
-Strategies for confronting existential risk-- high interest
-Potential biases in evolutionary psychology-- mid-level interest
-Taking ideas seriously-- extremely high interest
-Various community threads-- low/mid interest
-Quick explanations of rationality techniques-- extremely high interest
-Conflicts within the mind-- mid/high interest
Overcoming Bias front page:
-Personality trait effects on romantic relationships--...
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