Hi Ben,
I used the code straight out of the User Guide:
{exp:comment:form channel="standard_channel" form_class="comment-form"}
{if logged_out}
<label for="name">Name:</label> <input type="text" name="name" value="{name}" size="50" /><br >
<label for="email">Email:</label> <input type="text" name="email" value="{email}" size="50" /><br >
<label for="location">Location:</label> <input type="text" name="location" value="{location}" size="50" /><br >
<label for="url">URL:</label> <input type="text" name="url" value="{url}" size="50" /><br >
{/if}
<label for="comment">Comment:</label>
<textarea name="comment" cols="70" rows="10">{comment}</textarea>
<label><input type="checkbox" name="save_info" value="yes" {save_info} /> Remember my personal information</label>
<label><input type="checkbox" name="notify_me" value="yes" {notify_me} /> Notify me of follow-up comments?</label>
{if captcha}
<label for="captcha">Please enter the word you see in the image below:</label>
{captcha}
<input type="text" name="captcha" value="{captcha_word}" maxlength="20" /></p>
{/if}
<input type="submit" name="submit" value="Submit" />
{/exp:comment:form}
The major differences I see between what I have and what you have are the inclusion of some variables in my form. I’ll mark those differences below:
{exp:comment:form
channel="blog"
form_class="standard"
return="{page_url}#comments"
}
<label>
<span>Full Name<em>*</em></span>
<input type="text" name="name" value="{name}" />
</label>
<label>
<span>Email Address<em>*</em></span>
<input type="email" name="email" value="{email}" /><br />
<em class="note">Your email address will not be displayed publicly.</em>
</label>
<label>
<span>Comment<em>*</em></span>
<textarea rows="6" cols="20" name="comment">{comment}</textarea>
</label>
<label><input type="checkbox" name="notify_me" value="yes" {notify_me} /> Notify me of follow-up comments?</label>
<button type="submit" name="submit">Post Comment</button>
{/exp:comment:form}
To wit, I added the {name}, {email} and {comment} variables to the correct form elements and added name=“submit” to your submit button.
Give that code a whirl and let me know if you get the errors as expected.
Cheers!