YouTube only displays 20 comments or threads and you have to click "more" to read 20 other comments. There's also a page that only displays comments: click "all comments" below the video description and you'll be able to read the top 100 comments.
When you click "more", YouTube fetches 100 other comments. For example, the page lets you read 300 comments with only 2 clicks on "more", while the standard YouTube video page requires 14 clicks.
Here's an example of comment page that shows the top 100 comments. You can click the drop-down below "all comments" to sort comments by date. Another example here.