It’s because there can only be 5 levels of nested comments including the top. You can always reply to a less indented reply in the thread and make it clear who you are replying to if context requires. We could up this limit but it is not advised.
Thanks for your earlier kind words. All the feedback here is being collected and, after a break, we’ll play around with things on the test system this week. If we can come up with some modifications that Craig likes and help readers having a problem with this site then they will be implemented.
For those especially struggling with the font, be aware that things like noscript etc will block loading of the Google Fonts and a fallback will be used. The actual main font we use is the second most popular web font in the world but I know that doesn’t help for people who hate it 🙂