Anyone who has commented on a YouTube video recently will be aware that the commenting system has changed. It has been integrated into Google+, not a problem for those of us already on G+ but a nuisance for everyone else. It should be an easy change if you already have a G+ account but some people have had problems.
Most problems seem to involve cookies. Cookies are small files web sites leave on your computer. They can be useful as a way of remembering your preferences and the like but they can also be misused. One of the ways to avoid potential problems is to control the way cookies are set. Chrome has an option to disable 3rd party cookies, that is only the site you are logged into can set a cookie. This may seem straight forward but many sites use 3rd party sites to add functionality, usually quite legitimately like WordPress and Gravatar. Unfortunately the link between YouTube and Google+ is broken by setting this option.
If you use Chrome and find you can no longer comment on YouTube but have already set up a Google+ account check your settings. Go to Settings – Advanced – Privacy – Content Settings, under Cookies check the last option (Block third party cookies and site data) isn’t checked.