First off, you have to realise that the camera meters for 12-13% grey (a neutral surface that reflects about 12-13% light). Many use a 18% grey card and open up the exposure by half a stop, but you can (naturally) get 12% grey cards as well.
That is the general rule of thumb, but Canon had to complicate this further. If you switch to evaluative metering, IIRC the camera will assume that a certain portion of the upper part of the frame is probably sky, so it'll take into account that this part can be brighter. I believe they started doing that with the 10D?
For me, one of the most useful yardsticks is the sunny-16 rule. If you dial in aperture f/16 and set shutter speed to 1/ISO, then your exposure is set to a bright sunny day. (I never shoot at aperture f/16 of course, but find e.g. f/8 and 1/400s, ISO100 to be useful in many situations) More here: http://www.camerahobby.com/EBook-Metering_Chapter3.htm
IMO, if you have time to plan your shots, then you should shoot in M. Otherwise Av (or Tv in case maintaining shutter speed is more important) is the way to go. In places where the light doesn't change rapidly, go with M. Lecter, Sao and I were visiting the floating market, and we were constantly weaving in and out of the shadows, lecter in Av was way more productive than me in M. (he probably would've been anyway, but the mode played a part too)