This is somewhat unexpected, as it used to work in eZP 4.3:
1- enable extension
2- add attribute of "comments" datatype to a content class
3- click OK, you're done

Now step 3 produces an error, and unless you go to recaptcha.net to get a key, or alter ezcomments.ini, you will not be able to add comments to your classes.

While the intention is good, it does nothing to prevent the user from having an invalid configuration:

• the recaptcha keys in ezcomments.ini could just be wrong
• the recaptcha keys could be removed from ezcomments.ini aftre the class has been edited

So I think it should not become a blocking check, but at most emit a warning