I want to be able to reply to comments and answers directly, instead of having to comment again.
You use comments to reply to answers... as well as other comments. That's basically how comments work already. I'm not sure what you're getting at otherwise.
If you're talking about threaded comments, no, that's just going to overcomplicate things.
Stack Overflow is not a forum. Our job is to ask questions and provide answers to those questions. That's why comments are small, compact, and ignorable, while questions and answers are big and important. You should avoid attempting to have a long discussion with people. If you're not asking a question, asking for clarification on a question, answering a question, or trying to improve an answer to a question, then your commentary is not needed.
You can effectively reply to a comment using
Both of these require having commenting privileges. If you don't have that, then go earn some rep.