What's the difference between email spam and comment spam, exactly? Why don't online forums use Bayesian filtering to get rid of comment spam?

I haven't followed SpamAssassin in several years, but have any startups ever tried running their comments through it?

As you point out, it seems like many of their rules for emails would apply equally well to blog comments.

Doesn't Akismet work the same way as gmail's filter?

Yeah, you missed the point here.