At the moment I can "subscribe" to an OpenID user, but this appears to have no actual effect. It seems like it can't do anything meaningful in general, because OpenIDs don't have a stream of postings that could be displayed... however, there is an important special case, which is that many OpenIDs actually correspond to Livejournal accounts, or accounts on other services of this type. In the case of a Livejournal OpenID it happens that the ID points at a URL which is for a Web page whose metadata contains a feed URL. It seems like in such a case it should be possible to pick up the feed URL and syndicate it. That's what subscribing to an OpenID ought to mean.

So, I suggest:

(best) When someone subscribes to an OpenID, check the corresponding URL for a feed link in the meta tags, and if there is one, treat the subscription as a subscription to that feed. If there is no feed, generate some sort of error or warning.

(okay) If subscribing to an OpenID currently does something, then clearly document what it actually does.

(compromise) If subscribing to an OpenID does not actually do anything and can't be extended to do something reasonable, then make it impossible to subscribe to OpenIDs.

Over on the relevant comment thread on my own journal, ai_hua@livejournal suggests that it would be really nice if, in addition, comments made on her syndicated feed could be forwarded to her Livejournal account - but that obviously opens several other cans of worms, and it sounds like it would require Livejournal's cooperation in a significant way.