Sex, politics and religion do not belong in a professional environment. You are there to work, save the other stuff for after hours.

Oh lol, how north American of you. On the last two i mean.

If you compliment someone's attractiveness, you might be implying that they got to where they are due to their looks, which implies that they do not have the skills to get there themselves and that they are slightly dishonest.

Women used to only been seen as good for menial jobs and as eye candy unable to do real work but kept around for the menfolk to look at. By calling someone sexy you are reinforcing this old view, or implying that you might hold that view yourself.

If you'd like to read the comments on the blog post, I'm sure you'll find plenty of women, gasp, agreeing that it is sexist.

I'm certain the majority of HN believes OP's example to be sexist. I may also be sexist - I apologize. I also understand that things said in the workplace can be interpreted as reinforcing a sexist, though antiquated way of thinking - which would be a misunderstanding if applied to something 'I' say ;) The fact of the matter is that from a biological standpoint everybody is judging everybody on all fronts - your mind does not stop thinking your sexy co-worker is sexy just because you are at work. Ugly productive people have to work harder to prove their productivity than attractive productive people and that's not fair, but C'est la vie. If I were attractive AND smart (which I'm obviously not as I'm arguing this point, here) I would like to be judged on both fronts - if I were only smart I would only want to be judged mentally - if I were only attractive I would only want to be judged on appearance. Each case skews the playing field in my advantage. Though the truth, I fear, is that we are all judged on all fronts. I'd rather not delude myself and deal with reality. I will take all compliments as compliments in a positive light. Meanwhile everyone should chill out and be happy that they are not dumb and ugly.