As best I can tell, the available options are:

‘Subscribe by Email’ – which some blogs have
‘Reverse pingback’ which is a made up technology, but fairly self explanatory concept. Trouble is, it would generate a *huge* about of traffic over the net.
RSS comment feeds for specific posts

It can’t be hard to generate comment feeds per post on the fly, and it would fit into existing feed reader tools (though might require RSS readers to learn to cope with many more subscriptions).

Subscribe by email works fine too, but it’s nicer to have blog reading separate from email reading, since it destroys my productivity if I do both at once. Ahem.

The problem with all these is that you need every single CMS on the planet to implement it and every blogger to update their software. I don’t know how to work around that.