I close comments on posts that are more than 30 days old. This is why. The widget I used to have on the sidebar that posted recent comments was wonky at best, and it cluttered up an already full sidebar. What this means is that if you comment on a post that's, oh, over a year old, the chances you'll get a reply are insignificant. If you're hoping for a conversation, you'll be disappointed, and if you're taking a shot, no one other than me will notice, because it gets shipped to moderation and I deny them out of hand.
If either I or Amy have something else to say on a subject that we've blogged about--and we often do--we'll address it in another post rather than carry on a conversation that's long since died. If you're a regular visitor--and I hope you will become one if you aren't already--then you'll get a chance to add to the conversation then.
One other thing. We don't get many conversations going here, but I still have no problem deleting comments I find offensive, racist, sexist, or which don't deal with the subject of the post. I allow for drift, certainly, but I won't hesitate to slap down threadjackers or spammers.