Comment option #1 - Using a third party comment provider called Disqus. Cons: They require email confirmation before posting for the first time in order to prevent spam. You see mentions of "Disqus" website in confirmation email which may be confusing for users?
Comment option #2 - Using Facebook. Cons: Anyone without a facebook account couldn't comment. And this would probably require more work because Facebook code would probably need to be added to the bottom of every new blog to add the comment section.
In either case, the biggest con to both of these is these would replace the old shopify comment section. There'd be no way to keep the old comments if we switch to one of these new methods.