Anything much past that, I don't really get. I have no idea whether there are other vocations that pay at a (legal) sub-minimum wage with the expectation of making up the difference with tips. Here's a case in point, but first a little background: I am a man of simple needs. I have gotten my hair cut by people outside of my family only a handful of times. That was the case until I moved to college and didn't see my family for, like, four months (no one wants to see my hair uncut after four months...again (but that's a completely separate story)). Anyway, I went to get my hair cut at a SuperCuts* (or similar hair cutting joint (again, a man of simple needs)). I got my hair cut, paid for it at the price the "menu" said, and went on my merry way (that's kind of disturbing to hear from me, I'm not ever really "merry"). By this point, you've probably figured out what I neglected to do: pay the tip. Not knowing a tip was expected, I was in that "ignorance is bliss" state until I returned to my dorm and was talking with my roommate (I think, it was one of my friends freshman year) and he mentioned that tipping was expected for hairdressers. Well, needless to say, I felt like a(n) (what was that word I was talking about above?). It wasn't that I didn't want to tip the hairdresser, I just didn't know. I kind of get that one: I'm trusting them with the way my hair looks (and most people care about the way their hair looks) and I should pay them as well as I believe they've handled the job.
Next comes taxis. I don't think I was the first to pay for a cab ride of which I partook (it was a shared business thing, people aren't giving me free cab rides for no reason). The payer added a tip to the bill. I have since Googled* (yes, I'm a nerd, I know) and realize, apparently, that cab drivers expect tips.
That's all I know. I don't understand or see what similarities waiters, hairdressers, and cabbies have. I don't know who else should get tipped. Maybe you guys can comment and tell me.
Last point: why do only waiters get the tips. I really don't care much about their service (as long as it doesn't suck), I'm more concerned about how my food tastes. I should tip the cook. Not to mention the fact that busboys (or busgirls) have to deal with my dirty dishes and napkins and table crumbs (maybe not mine, but the general public's table crumbs (I try to keep my area relatively clean, out of respect for them)).
Also, and this has nothing to do with anything I think, but it's interesting to ponder: I remember a Colin Cowherd* (funniest/smartest man in sports radio) bit about the fact that we should tip the waiters/waitresses and cooks before, not after, the meal. If I want great service, I should give them the motivation beforehand, rather than making them wonder whether I'm going to be that (word from above) and not tip well.
* Apparently, this post is brought to you by Tommy Boy, SuperCuts, Google, and Colin Cowherd.