I dine alone often, and now that my daughter's going to college, even more often. I like to be treated like any other patron, but many restaurants fail to greet single diners with the same respect as persons dining in groups.
I understand that when someone walks in the door, it is important for the restaurant to know if you're dining alone, but "party of one?" should be said in the same neutral or upbeat way as "party of four?" would be. Often I feel that the host/ess is passing judgement on my choice to dine alone, or sadly empathizing with my plight. I know I'm dining alone, really. You don't need to make an issue of it.
Worse: "Only one?" (Once I actually heard "Only two?" from a hostess. What? Two is not enough?)
Worst: "May I help you?" This always sounds like they've sized you up, you are not a potential diner, and must be here to sell something or apply for a job. I only hear this in restaurants, not takeouts, too.
I won't get into what seems to be bad (or no) service that individual servers may give single diners anticipating poor tips. This seems to be a problem of individual servers, and only occasionally happens.