On the one hand, I think it would be great for some of the bigger threads, that often go to hundreds of comments, many of which say the same thing over and over again. It could do a lot of keep those shorter, more insightful, and more on-topic.
On the other hand, what about the smaller threads, that tend to get one or two dozen comments, if that, and probably not many page views? Would discussion there become essentially impossible unless a >1k-er deigns to drop in and bless a few of the posts with some endorsements? Has anybody actually checked what proportion of users are >1k and how willing they are to drop in on every thread on the site and endorse comments? Or constantly reload bigger threads and scroll all around to see new comments for that matter?
If it were up to me, I'd do a sliding scale. Something like no endorsement/auto-endorse all for under 20 total comments in the thread. At >50 comments in the thread, users with under, say, 100 karma need endorsement. At >100, you need maybe 250 karma, 200 posts 500 karma, etc. Maybe users under 5 or 10 karma always need endorsement. Maybe some formula to compute it that you can tweak the factors on as needed if there is too little discussion or too much fluff.
Another point - say I make a comment that sucks. Oops, happens to all of us sometimes. What happens then? Does it stay unendorsed and stuck forever? Can I just delete it, or will it auto-delete or something? Does it get downvoted without ever being endorsed?
Hey wait a second, I think we're just getting closer to slashcode here...