I know what you're talking about but it doesn't apply to comments. The only thing that automatically adds up and downvotes to comments is what they call 'fuzzy comment karma' which is to prevent bots from figuring out too quickly that they have been shadow-banned, since to them it looks like they are getting 2 downvotes here and 3 upvotes over there.
The system has been active longer than the post was old and it's just and example, if you read through comment threads back then and now, you will notice the difference without having to rely on karmic metrics.
Oh, yes, completely agree. It not the karma scoring that hurts, but rather than no discussion is considered acceptable.
it is best to simply accept downvotes without explanation as background noise
Sadly, yes.
WRT Reddit what I've heard on the site is that they have an obfuscation system that adds downvotes and hides the current voting position. It would be impossible thus to see that a story had 91 upvotes and no downs as apparent downvotes are automatically added. It's supposed to be part of the system to prevent people from gaming karma scores.
Note: the following unexpectedly turned into a bit of a rant, with a dose of whining thrown in:
Many of us "old-timers" think that downvoting without giving a reason is mostly bad form (the exception is for a rude or obnoxious comment). I've personally noticed an increase in this behavior.
I believe that doing so decreases the user experience. I've had plenty of disagreements over the years and have almost always learned something from them (even if it just required me to do a bit of soul-searching). Posts would be painstakingly composed (one of the big complaints we had was that the post-page timed out while we were composing- so we had to result in using a separate editor to compose, then click "reply" and copy-and-paste.
But now-- why bother? I've been around a while. I'm on my second (third?) career. Not a hacker extraordinaire by any means, but I'm well educated technically and have maintained my involvement with a broad array of interests (I like to learn new things). I've seen some things, and I'm willing to share- and even be challenged in my own conclusions about them.
But when I make a post that the "average HN-er" (age 25, so I hear) may not think is very smart and I immediately get downvoted with no reason given, it sends a signal that HN has changed in a very dramatic way.
The site is far less valuable than it used to be.