I believe that choosing correct paper has the biggest impact on the finished painting, making certain effects much easier to achieve.
There are four basic characteristics associated with watercolor paper: surface texture, weight, whiteness and hardness.
Additionally, there are three types of surface finish: cold pressed, hot pressed and rough.
Cold pressed paper has a subtle surface texture between moderately smooth and slightly rough. I use this type for most of my paintings.
Rough paper is not pressed at all, and the texture can be very rough on heavier paper. I do use this type of paper occasionally and by controlling water and color I can get very interesting effects on that paper.
Hot pressed paper is pressed between hot rollers to give a smooth surface. I know that a lot of artist like this paper, but I hardly ever use it.
My favorite paper is Arches 140Lb (300 gsm).