I must be stupid or this site has a weird interface. How do I reply to someone's question so that it shows as a threaded discussion item instead of a regular answer? I don't see any button or link to do it.
The Stack Exchange user interface is completely different than that of a traditional forum for a very specific reason: These are not traditional forums. Instead, Stack Overflow and the network of Stack Exchange sites are Q&A, or Questions and Answers sites.
The problem with many forums is that discussions tend to create a lot of noise and bury actual answers to problems. Just think about the last time you read through 4 pages of discussion only to find out that no one's suggestions solve your problem or that the link included on page 3 is now broken.
On Stack Exchange sites, the answers that are most helpful or that solve a problem are voted to the top. This helps a visitor quickly determine if there is a solution on that page and either benefit from that answer or quickly move on.
Furthermore, comments are known in the Stack Exchange community as second-class citizens. Although not always the case, their existence is intended to be temporary. Comments are intended to seek clarification on a question or help a user improve an answer. They are not intended to hold extended discussions or debate.
If a question is missing lots of detail, then it may need to be edited and improved, or flagged for moderator attention and either closed or removed.
Commenting is available at 50 reputation. You're pretty close :). See here.