As you've noticed, you need more reputation points to comment on someone else's post than to ask or answer a question. Why's that? Because new users don't always understand how this site is different from a web forum or a mailing list. They don't always realise that Stack Exchange sites have questions and answers, rather than threads and replies. Sometimes, new users think it's appropriate to add a comment to a question to say, "I'm having the same problem too," or to "bump the thread". That's not how this site works. Comments like that don't add anything to the site, so we don't want them. If you're having the same problem as someone else, you can star the question to be notified of new answers.
I notice that you used the term "thread" in your question. Perhaps you've fallen into the same trap: it's an easy enough misunderstanding to make until you read the FAQ or use the site for a while. That misunderstanding is exactly what the privilege system is for. Once upon a time, it was customary to lurk on a mailing list or BBS (that is, to read the site without posting), so that you can learn the conventions and the culture of the site rather than posting in the wrong place. The privilege system exists, in part, to help keep that custom alive here.
It's up to you whether you participate or not. As you say, there are are plenty of web fora out there if that's all you want. But if you stick around, I think you'll find, as I have, that Stack Exchange's reputation and privileges help keep the quality of discourse more productive than you find on the average forum or mailing list.
