Since Youtube/Google changed their subscription page layout, a LOT of users have complained that it's very hard to track what people have now posted, or what subscriptions they have/haven't watched yet.
Youtube does have RSS feeds for user subscriptions, so you can subscribe to it as an RSS feed:
Having this as an option (just enter your Youtube username, it goes and gets the matching feed) would open this up to a lot of people. Note that you only need the Youtube username to get the subscription feed, so doesn't seem to require you to login to Youtube. Some users though don't know their Youtube username (as they've connected their Youtube and Google+ accounts, and since forogtten, or it's been auto-generated from G+), so there may be a way to fetch the username once the user is connected via G+.