Whether a user account is unregistered or registered is a technical detail that should not have any influence on how they are treated.
Anyone is free to decline to answer or curate questions from unregistered users, or questions from users whose name starts with A, or questions from users whose avatar includes more green pixels than blue pixels. That's because everyone is a master of their own time and involvement.
On the other hand, to vote differently based on whether a user is registered or not would violate the one rule of voting: vote for the post, not for the user.
If you notice that a user is having trouble because their account is unregistered, then it's ok to guide them towards registration. Don't just say ”you should register your account“. Most users don't even know what that means, and even if they knew, they'd have no reason to do what you say. Any comment that says “you should register” must include two pieces of information: how and why. For the how, link to How do I create an account?
from the help center. Note that it doesn't say “register your account”, it says “creating an account”. The terminology around accounts is a bit confusing. For the why, explain how this would solve whatever problem they're having, e.g. that this will allow them to upvote answers (in response to a comment saying ”thanks“).
If a user has lost access to their original account and as a result posts an answer instead of commenting or instead of editing their original question, point them to I accidentally created two accounts; how do I merge them?. Since users aren't notified of comments on a deleted posts except for comments left by a moderator shortly before or after deletion, it's usually up to moderators to leave such a comment, but in case the user does visit the site before the answer is deleted, you can speed things up by leaving a comment in addition to flagging.
A comment like “Here comes another unregistered user (I was planning to write an answer...).” is unhelpful because the target wouldn't even understand what you mean. It's also rude because all the target will see is ”I refuse to help you for some obscure reason, even though I would have helped someone else“. Moderators delete such comments on sight, and repeatedly posting such comments may lead to sanctions.