This plugin doesn't work on vanilla 2.1 stable

geexis

I cannot post any discussion or question while using this plugin and the characters count won't work either.

Csabbencs

I don't know why it popped up just now, but the reason is that after clicking submit TinyMCE never writes back the value you entered in TinyMCE editor's body to textarea html's body. This causes that updated values never reach postcontroller. you would always see the old value.
There might be a better solution, but you can solve the issue by writing back the new value after every submit into textarea for which - unfortunately - you have to modify core code at post.js after #DiscussionForm :submit and #CommentForm :submit:
if(typeof tinymce !== 'undefined') $.each(tinymce.editors, function() { $("#"+this.id).val(this.getContent()); });