Comment Moderation Does Not Scale
In computer systems, "scale" means growing bigger as in blog traffic growing bigger, etc.
"Comment moderation" means having a blog that does not automatically publish visitors' comments. But rather, that all comments must be approved by the administrator before it gets published. The act of discerning whether a comment is worthy to be published or not is called "moderation". Moderation also means filtering out spam comments.
Example of Scaling
If a blogger has 30 posts, that would not be too big of a problem to manually moderate all incoming comments. However, if the blogger later has 300 posts and then later has 3000 posts, then comment moderation may become a chore and may take considerable amount of time.
This takes into consideration that some comments are quite long to read, the administrator must also check out the visitor's website link and/or profile to determine if the visitor is a spammer or not, etc.
Even if, say hypothetically, that each posts gets one comment per month, that would mean that an administrator must moderate 3000 comments each month for a 3000-post blog. If it take one minute to moderate one comment, that would mean it takes a blogger 50 hours each month just to moderate comments. That's is an entire working week.
I know that these hypothetical numbers are in the extreme on the high end. Most bloggers are not at the level that they would encounter these large numbers. These numbers are just nice round numbers to make the calculations easy to see.
However, it does demonstrate the idea of "scaling". That is... What is completely feasible on the small scale, may become unfeasible in the large scale. That is what is meant by a system that "can not scale".
On HubPages, I am planning on publishing 1000 hubs within a span of five years. With a hundred or two hubs, moderating comment will be do-able. But not when it reaches the 1000 hubs mark. More Hubs means more comments, and it can become quite time-consuming to moderate.
Some Large Blogs are turning off comments
To be sure, there are popular blogs where those large numbers are reached.
Most notably is ZenHabits.net, the top blog in 2010 as reported by Time, turned off comments. Its owner Leo Babuta explains ...
"There was too much comment spam, resulting in huge headaches for me. Seriously, it took up a lot of my time."
Steve Pavlina turned off comments back in 2005 when comments reached more than 10 per day and on some day as much as 100 comments. He says that "The main issue was the opportunity cost — the amount of time and energy it takes to handle comments and what other choices I have for investing that time and energy."
Actually, It May Scale
Some would argue "why moderate comments" -- just let all comments through. Yes, that would make it scale. But that is not advisable. The Webologist warns bloggers why they should always moderate comments and never auto-publish. Because it can have legal ramifications such as when a libelous comment is left on a blog post.
There are a lot of spam comments. And spam filters does not work 100% correctly. Also you have to monitor what people say before publishing it. Because there can be legal ramifications due to stuff people put in comments.
Actually, it might be possible to make comment moderation scale, if the blogger hires someone to moderate comments for him/or her. However, there are problems with that as well.
First of all, where are you going to find someone who is willing to moderate comments for you part-time? Second, it costs money. And third, it is not always practical.
As Pavlina's contact form says "I could hire an assistant to handle this for me, but that seems pointless since most messages people have sent me are very personal."
So you see now why comment moderation may not be able to scale as your readership grows. Another article that you might be interested in reading is Why I Do Not Have Comments Turned On on some articles.