in the loading screen at the top you can see whether you and the opponent are evenly match or not, by ladder points, if both teams are evenly match the score will be 11 points only (win or lose),

if you are slightly favored(means you have more points) you will win only 8 points but if you lose, you will lose around 16 points while your opponent gain 16.

and no you don;'t get more points from streak.

bonus pool is points for not playing for a limited amount of time.

Yup. It's fixed. The only thing that helps you to gain more points is the bonus pool, that is to allow you to catch up with other players if you've been inactive for a while.

no point, you'd gain more by playing more anyways. the bonus pool is meant for people who cant keep playing and give them a bonus so they dont get left too far behind.

also, per division within a league has ~100 players. and the amount of score helps determine how you are ranked within the division. and if you get promoted or demoted, your score goes back to zero within that league/division

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Added on August 12, 2010, 3:31 pm

when you win/lose a game

I too have a question regarding the promotion and demotion within the league.
Lets say i am involved in a 2 v 2 team. How do i get promoted / demoted.

Do i have to be num 1 for a certain period of time in order to get promoted?

score, win/loss, how you win/lose, APM, etc etc

many many many factors that decides this by bnet system that is not revealed by blizzard. you want to go up, just be good. if you're not good, you go down. simple as that.