I have the following problems with blog comments:

• The interfaces are all different, and all suck (unlike my blog posting interface, which works exactly as I wrote it to).
• The markup is all different and all sucks (unlike the one I use for my blog, which I carefully chose).
• Once I type my entry into the void of a blog's comment box, it's no longer mine, it's not well associated with me, my name is not provably on it (openid helps with this one), it's not archived somewhere I control.
• People posting comments to my blog don't help spread links to my blog the way that people blogging about my blog do.
• There's generally no rss feed to follow comments; if there is, it's too annoying to subscribe to it.
• Since comment systems are a nasty little appendix on the side of blogging, no neat new tech that's developed based on blogging is likely to improve them.

So if I have something important to say, I'm likely to blog it even if I do post it as a comment too.

Compare and contrast with email..