LMP,
While I don't have a list of people I rep, I find myself repping just about the same people often. Nilp is just hilarious and has earned a couple reps for his 'work' in Mith. Undome seems to stay mostly in the RPG sections yet she's an excelent writer and I rep her from time to time for her posts. In the Books/Movies sections it's more open and I usually rep anyone who has some valid, well argumented points.
I like repping newbies as well, specially in the discussion forums. Not that I'm an "old timer" but I like it when someone new comes in and adds to the discussion.
About why do I rep people? well, either a really funny post or a good argument. I think I am more easily persuaded by a good argument in a discussion than a joke.
Eomer & Rune,
It's crazy, but same here. I can't recall which post it was, but I remember really spending a lot of time making a good post for one of the book discussions and I was so confident I had made a good post I just 'knew' I'd be repped. Yet I was not
at least until
Estelyn digged it up about a month later!!
I try hard not to rep the poster too often, but rather the posts. Still, I find that I rep more often those I have PM'd or Roleplayed with.