"There is a world of difference between saying to a colleague “you look nice today” and “your arse looks good in those trousers”
In theory, I agree. But actually, I think it is inappropriate in most cases because the person on the receiving end may or may not be comfortable. Before everyone starts shouting about Snowflakes, my point is that workplaces are usually hierarchies. A comment about appearance is, usually, entirely pleasant and non threatening with no ulterior motive. BUT.... unless you are personally also friendly with said colleague, how do you know?
Don't get me wrong, I wouldn't be rushing to HR if someone comments on appearance. It just feels to me like it assumes too much and it's unnecessary. The 21 year old intern might not actually be in a position to feel comfortable. She might feel self conscious if she's being noticed. She might not know how to respond.
I worked with a young PA once. Her clothing started to become inappropriate in the office. A few "jokey" comments were made on her appearance and, as it turns out, a LOT of comments were made behind her back. Culminating in an unpleasant direct confrontation during a performance review that came out of nowhere (for her) and made her understand people had been talking. As it turned out, I, as the senior person on the team, had also noticed her clothing changing. And while she didn't report to me directly, had quietly taken her aside to discuss it. It turned out she'd been gaining weight but hadn't wanted to acknowledge it and buy new clothes. Subsequent to our conversation, her clothing improved. And by the time it was brought up in her performance review, her dressing had reverted to entirely appropriate. Which her line manager was forced to agree when the PA pointed it out. (I found out all this only because she then approached me to thank me and told me that if I hadn't had the informal, yet official, chat, it would have been more severe and upsetting). But her upset over the whole thing didn't fade and she subsequently resigned.
The problem was the informal comments, made to her and about her, created a very unpleasant working environment for her. Obviously, this isn't the same as people regularly commenting on how nice someone looked, but in the same office, a very stylish women got a lot of comments. She told me years later that it put a lot of pressure on her as she felt if she wasn't perfectly turned out every day, people would notice and/or comment.
It just seems to me that it's not necessary. It's not harassment or whatever, but why even go there?