I have one blog that is starting to gets a great deal of traffic. It is an American Idol blog and the fans are leaving great comments. However, some of them are quite long like in this post. Should I consider limiting the length of the comments they can post or should I leave it as it is?
Till the time comments are genuine, related to the post and adds value to the post, I don't see a reason why you should limit the length. The comments you are showing are well within appreciable limits and can be read easily. Moreover, your theme handles long comments easily. You can limit the number of comments per page by paginating them but limiting their length is not a good idea currently unless you start getting comments in 1000+ words. Few long comments don't look bad and shows the level of engagement on your blog.
Limiting comments would be really dumb. But I suggest you to split comments to pages, because current load time is not the greatest. Be happy that people comment
if comments are according to the topic you should approve them other wise delete all spam or install a plugin that remove spams comments automatically .
Don't stop the comments until they are genuine as comments will count as a change on your post that will help google crawler to crawl that post frequently.. and if you are worried about their bad appearance just split them into pages and for spamming you can use captcha's and akismet both..
How do I split it into pages. Also I did read where the comments are like fresh content for Google to crawl.
Read this : http://codex.wordpress.org/Function_Reference/paginate_comments_links or use this plugin http://wordpress.org/extend/plugins/paged-comments/
You can alternatively limit the comments by checking "Automatically close comments on articles older than xxx days". There is an option in discussion settings about this.