SC2 Ladder Analysis Part 1: http://www.teamliquid.net/forum/viewmessage.php?topic_id=118212
SC2 Ladder Analysis Part 2: http://www.teamliquid.net/forum/viewmessage.php?topic_id=142211
SC2 Ladder Analysis: Division Tiers: http://www.teamliquid.net/forum/viewmessage.php?topic_id=169830
SC2 Master League Information: http://www.teamliquid.net/forum/viewmessage.php?topic_id=183626
Q: What do the different symbols, like the exclamation point, red arrow, and green arrow mean when viewing a division?
A: An exclamation point means that the player has been placed in that division but not played any games last week. A green arrow means that the player has increased in rank within the division since last week. A red arrow means the player has fallen in rank within the division since last week. A dash (--) means no change in rank since last week despite games played. You can hover over each icon in-game to view the player's previous rank or division join date from the in-game ladder browser.
Q: Will the ladder be divided into seasons?
A: Yes, eventually. The length of a season has not been announced by Blizzard, but all ratings are expected to be wiped at that time. MMR will persist across seasons.
Q: I'm only in Silver, so why am I being matched against Platinum players?
A: Two possible answers: either your MMR is too high for your current league, or your opponents' MMRs are too low for their current league. Look at your match history. Are you commonly being matched against players from higher leagues? If so, you have an opportunity for promotion. Look at your opponents' match histories. Are they commonly being matched against players from lower leagues? If so, they are at risk of demotion.
Q: If I've never played 1v1, but I'm 2v2 Diamond, who will I face in 1v1 Placements?
A: Your performance in other brackets is considered when initially seeding your placement matches. In this case, you'd likely be paired with a Diamond player to start.
Q: What does Favored/Slightly Favored/Teams Even mean on the loading screen?
A: Favored status compares your opponent's hidden MMR with your displayed rating. If his MMR and your displayed rating are similar, it will show Teams Even and the match will be worth 10-14 points. If his MMR is slightly higher than your displayed rating, it will show your opponent as Slightly Favored and the match will be worth 15-19 points for a win (5-9 for a loss). If his MMR is significantly higher than your displayed rating, it will show your opponent as Favored and the match will be worth 20-24 points (0-4 for a loss).
Q: Is it possible for both players to see the other as Favored?
A: Yes, because it compares the opponent's MMR to the player's displayed rating. This is common when both players haven't played many games, meaning their MMR is higher than their displayed rating.
Q: I'm in Bronze with 0 rating and a poor win ratio. I'm only getting 2 or 4 or 6 points -- including bonus pool -- for a win, but losing 22 points for a loss! Why is this happening, and how will I possibly get above 0 rating as long as this is happening to me?
Fixed in 1.3
A: This one was tough to figure out because it doesn't make a whole lot of sense on the surface. Point gains and losses that are that extreme can only mean that you're Favored in every single match, but how is that possible if you're already in Bronze with 0 rating? The answer is that your MMR can go below Bronze 0. Because your rating -- which is zero -- is higher than your MMR which is somewhere far below zero, you will constantly be Favored. Because of the way MMR gains work, however, as long as you maintain a positive win ratio, you will eventually find matches where you are Slightly Favored and ultimately Teams Even and beyond. At that point, you will start to gain more points than you lose.
Speculation: This may be a remnant from Beta, either from when Copper league was removed or when players started at 0 rating when they used to start at 1000 rating. It could be that Bronze 0 is equal to Beta's 1000 rating (and therefore Bronze 0 = 1000 MMR).
More info: Allowing MMR to dip below the minimum point value is one way to help ensure that inflation beyond the bonus pool does not occur. The player who loses will have his MMR dip below Bronze Zero, which will cause him to be Favored in every matchup and therefore, because points drift toward MMR, he will remain at zero. This means that any points that are gained are quickly taken away by any losses, thus vacuuming up any points that would have been not really "earned" by the points floor until his MMR rises above the equivalence of Bronze Zero. This stops the production of extra points from the floor of 0.
More broadly, your points drift towards your MMR. Because of this mechanism any inflation that occurs will be counteracted by the game outcomes. That is, if point inflation in fact existed, then your points would eventually outpace your MMR and you would then start gaining less points on a win and losing more points on a loss, thereby negating any "inflation" that might occur. Note that this assumes that MMR does not inflate.
EDIT: Oh hey looks like this is a bug: http://us.battle.net/sc2/en/forum/topic/627980513?page=2#29
...what you're experiencing is indeed a bug and we're in the process of developing a fix. We wanted to make sure that you and any other players experiencing this don't get discouraged. The fix will require you to keep playing once it's applied for it to resolve the cycle you're in, so we hope you'll continue doing just that.
Promotion and Demotion
Q: Do I need to be first in my division to get promoted, or last in my division to get demoted?
A: No! Your rank is a result of your displayed rating and is not relevant to any promotion chances or demotion risks.
Q: Do I need to reach a particular point value to get promoted?
A: No. Promotions happen independently of your displayed rating.
Q: I've been at rank 1 in my division forever, why haven't I been promoted?
A: Take a look at your match history. If you're commonly facing opponents from your league, then you're right where you should be. If you're commonly facing opponents from higher leagues, you may be promoted once your MMR's moving average reaches a certain league threshold.
Q: I won 10 games in a row! Why am I not promoted yet?
A: Your MMR and moving average may not have reached stability, or the system is simply unconfident about you. Expected outcomes cause you to stabilize the fastest. Your MMR will continue to rise until you hit your skill ceiling, at which point your win ratio will decrease and your rating will start to stabilize.
Q: Do I need to lose 7 games before being promoted?
A: No. There is not much point to gaming the system. Play as you would normally and if you belong in Diamond you will be promoted within a very reasonable number of games. People are regularly promoted with < 7 losses. People like CauthonLuck are significant outliers and you will almost certainly not have his problem.
Q: I'm in Bronze. Is it possible to get promoted directly to Gold?
A: Yes. It's possible to go from any league to any other league, wherever your MMR and sigma stabilize. Viewing your match history and the profiles of your opponents should clue you in to where you're likely to be promoted next.
EDIT 3/14: Internal changes to the ladder system appear to have prevented "league skipping". The system is a little more aggressive about promoting players into new leagues, now.
Q: I just lost a game but I got promoted, how is this possible?
A: The system evaluates a trend of games, not just the most recent game. The answer is always the same: your MMR and sigma have stabilized and your moving average has come to rest within the boundaries of a higher league, and that loss contributed to your stability because it was an expected outcome.
Q: Is it possible I haven't been promoted because my APM or end-game score isn't high enough?
A: Both factors are irrelevant because they can be gamed. The only contributing factor to promotion is the end outcome (win or loss) and the skill level of your opponent (his MMR). This is confirmed by Blizzard's Leagues and Ladders FAQ, found in a sticky at the top of the Multiplayer and eSports forum.
Q: How do I increase my bonus pool? Do I have to not play?
A: Your bonus pool increases at the same rate regardless of whether or not you're playing. The rate of gain is about 1 point per 2 hours. More accurately, 1 point per 112 minutes.
Q: If I start a new team, won't my bonus pool be smaller than teams that have been active for a month?
A: The bonus pool total is the same for everyone. If a team was created last month and has accumulated 600 bonus pool, your new team will start with 600 bonus pool. The total bonus pool accumulates from launch day, meaning regardless of your account creation date, division join date, or whatever, it will be equal to everyone else's total.
Q: Is the bonus pool considered in any calculations? How can that be accurate if it constantly inflates ratings?
A: It has been confirmed that the bonus pool is not factored into any MMR or displayed rating calculations.
Q: I haven't read the Ladder Analysis threads linked above, but I totally will, I promise. MMR? Sigma? What do these terms mean?
A: MMR stands for Matchmaking Rating, and it's a hidden value that determines your opponents. MMR fluctuates greatly based on your wins and losses to various players, far more than your displayed rating. Sigma is the statistical symbol for standard deviation, and it represents an uncertainty factor (because not everyone plays at the same skill level all the time). Sigma determines your range of opponents.
EDIT 10/25/2010: Made several clarifications in light of information gained from Blizzcon 2010.
EDIT 3/14/2011: Noted that "league skipping" is no longer possible and that system promotions are slightly more aggressive than before.