I eat in restaurants four nights a week (maybe not as glamorous as it sounds). I'd say the number one gripe is service. Because I'm on my own, I'll either be treated like a star, or forgotten completely. I try mix up where I go, but because it's essentially a part of my work day, I tended to go to places that accepted AMEX (though we've now changed cards).
I hate being asked how the meal is, if I don't like something, I will let you know. And leaving an empty glass for too long. Honestly the number of times after an early start I will go in, order a beer and a main, knock half the beer off before the food arrives and finish the beer during the meal. 99% of the time if they ask me if I'd like another beer while I still have decent amount to go, I'll have one. But I rarely have dessert so it's no good asking me once my meal is almost done!