It sounds like you are asking how to deal with it in the moment, not in the long term. I'll try to help with both.

Why would someone say such a thing?

Knowing how to react is going to require understanding why a person would say something like that.

• They may have been uncomfortable and were carelessly expressing their feelings, either rightly or wrongly.
• They may be entirely wrong and are simply trying to tear you down.
• They may have no regard for your feelings and were calling it as they see it.

In the moment

Reacting appropriately in the moment is something that comes natural to some people and takes endless training for others (like myself).

If you are simply so surprised that you are speechless, I would either do as you did, thank them and leave, or simply ask a question: "Can you please explain? I'm not sure I understand."

If they are asserting that others share their opinion as well, I wouldn't hesitate to ask them to back that up. When it's about something negative, they really shouldn't be speaking for others, anyway.

If you believe they've expressed all they can and you'd like to not dig into it any further, simply apologize in a polite, but ambiguous way: "I'm sorry that what I've done has caused you to feel that way, I'll consider that in the future."

Unfortunately, in a professional setting, there's probably not much benefit from letting someone know that something was hurtful. Taking the high road will be seen as gracious by better people, and a hurtful person is probably not going to recognize the problem. A careless person could end up simply blaming you for taking offense.

Afterward

The only approach is to remove your bias and be rational.

*** The feedback could be completely wrong;** after all, smart rational people are usually also tactful. If you consider the feedback carefully, solicit some opinions, and can't find anything you could have done better, then I would move on and don't let it hurt you.

• The feedback could be correct, but only from a certain perspective. Some people dislike overly-casual people. Others may really like casual people. So, you may not need to change anything. Often, those people that dislike overly-casual people believe that everyone else sees the world as they do, when that's not the case.

• Maybe you could have done something better, but the feedback was expressed in a clumsy, inaccurate or overly-hurtful manner. In this case, maybe you could find something you said that you shouldn't have, and you can be more careful in the future, but this person was equally inconsiderate in expressing something they weren't prepared to communicate constructively.

• The feedback is correct but blunt. Hopefully this isn't the case; From the professional and thoughtful manner in which you asked this question, I would guess it is not.

It sounds like you are asking how to deal with it in the moment, not in the long term. I'll try to help with both.

Why would someone say such a thing?

Knowing how to react is going to require understanding why a person would say something like that.

• They may have been uncomfortable and were carelessly expressing their feelings, either rightly or wrongly.
• They may be entirely wrong and are simply trying to tear you down.
• They may have no regard for your feelings and were calling it as they see it.

In the moment

Reacting appropriately in the moment is something that comes natural to some people and takes endless training for others (like myself).

If you are simply so surprised that you are speechless, I would either do as you did, thank them and leave, or simply ask a question: "Can you please explain? I'm not sure I understand."

If they are asserting that others share their opinion as well, I wouldn't hesitate to ask them to back that up. When it's about something negative, they really shouldn't be speaking for others, anyway.

If you believe they've expressed all they can and you'd like to not dig into it any further, simply apologize in a polite, but ambiguous way: "I'm sorry that what I've done has caused you to feel that way, I'll consider that in the future."

Unfortunately, in a professional setting, there's probably not much benefit from letting someone know that something was hurtful. Taking the high road will be seen as gracious by better people, and a hurtful person is probably not going to recognize the problem. A careless person could end up simply blaming you for taking offense.

Afterward

The only approach is to remove your bias and be rational.

• The feedback could be completely wrong; after all, smart rational people are usually also tactful. If you consider the feedback carefully, solicit some opinions, and can't find anything you could have done better, then I would move on and don't let it hurt you.

• The feedback could be correct, but only from a certain perspective. Some people dislike overly-casual people. Others may really like casual people. So, you may not need to change anything. Often, those people that dislike overly-casual people believe that everyone else sees the world as they do, when that's not the case.

• Maybe you could have done something better, but the feedback was expressed in a clumsy, inaccurate or overly-hurtful manner. In this case, maybe you could find something you said that you shouldn't have, and you can be more careful in the future, but this person was equally inconsiderate in expressing something they weren't prepared to communicate constructively.

• The feedback is correct but blunt. Hopefully this isn't the case; From the professional and thoughtful manner in which you asked this question, I would guess it is not.