Great extension. I just added it to my default instalation. I think it could be even better, if it would count the title as well and show the recommended values for the title, meta description, and keywords.
I hesitate to make suggestions on the length within the plugin output, simply because it is a moving target. Each of the search engines have their own preferred length, and those lengths are arbitrary (there is not a standard that defines them), additionally - those lengths may change as the search engines adjust their SERP page layouts, or their indexing techniques.
Character counting in the title is a good idea. It will be in the next version.
Now, I might consider making a setting for each of the inputs (title, description, and keywords) which allows an administrator to define a target maximum length - which would somehow alert when that length is exceeded.
Thanks for the suggestions and the review!