No, you don't have to registrate. You can play the QuickGame without an account, too. But in order to participate in the Daily Challenge you need an account.
Yes, all services on sudokular.com are absolutly free.
You need a feedreader which supports the RSS 2.0 standard. All popular feedreaders like Firefox, Thunderbird, Google Reader support this standard.
Fair comments are always welcome. Simply fill out the contact formular and send us your opinion.
With an OpenID account — which you can get for example on myopenid.com — you can sign in with only one single password on all websites which are supporting this service.
Yes, on sudokular.com you can sign in with your OpenID account.