I found a PDF that shows all of the FICO Reason Codes. These are the codes FICO gives to lenders (and myFICO subscribers) to explain why a score isn't higher, so that a lender can explain to an applicant why they were turned down for a line of credit, and so an applicant can know what to improve. Here's the List. It shows which reason codes are used for different FICO models. The NextGen model isn't used by many lenders (PenFed is an exception), and many codes are only used for mortgage, auto, or other industry-specific scoring. If you just look at the codes used in the EQ/EX/TU classic base scoring models, all of them basically fit into these categories:
- Derogatory marks (delinquency, collections, public records)
- Not enough credit history (aged and recent, revolving and installment)
- Balances too high
- Too many recently opened accounts and inquiries
- Too many consumer finance accounts
I don't think any of this is shocking, but it also helps to know what isn't on the list. For example, not having a store card isn't on the list. ("Lack of recent retail account information" is a NextGen code, and some have reported getting something like this as a reason, so many people then believed that having a store card improved your credit mix.) Also, it's notable that for inquiries, the specific wording is, "Too many inquiries last 12 months." That seems to be when inquiries stop affecting your score (though their affect on scoring is sometimes exaggerated in the first place). And having too many credit cards ("too many bank/national revolving accounts") seems to be an issue for EQ and EX, but not TU.
This list isn't necessarily all the knowledge you need. For example, people report that their scores seem to be better if they have at least one or two more revolving accounts than installment accounts, something not obvious from the codes. How each of the factors are weighted is important, which is broadly explained by FICO as 35% payment history, 30% amounts owed, 15% length of credit history, 10% new credit and 10% credit mix.
For the indiustry-specific scores, like mortgages and auto loans, the reason codes basically boil down to "you don't have any history with this type of credit," "you have high balances, recently opened accounts or derogatory marks with this type of credit," or repeats of reason codes for the base models. Again, nothing terribly surprising.
Here's the full list of reason codes for the classic, base EQ/EX/TU scores:
- Account payment history is too new to rate
- Amount owed on accounts is too high
- Amount owed on revolving accounts is too high
- Amount past due on accounts
- Derogatory public record or collection filed
- Lack of recent bank/national revolving information
- Lack of recent installment loan information
- Lack of recent revolving account information
- Length of time accounts have been established
- Length of time revolving accounts have been established
- Level of delinquency on accounts
- No recent non-mortgage balance information
- No recent revolving balances
- Number of accounts with delinquency
- Proportion of loan balances to loan amounts is too high
- Serious delinquency
- Serious delinquency, and public record or collection filed
- Time since delinquency is too recent or unknown
- Time since derogatory public record or collection is too short
- Time since most recent account opening is too short
- Too few accounts currently paid as agreed
- Too many accounts recently opened
- Too many accounts with balances
- Too many consumer finance company accounts
- Too many inquiries last 12 months
- Number of established accounts (EQ/TU only)
- Proportion of balances to credit limits on bank/national revolving or other revolving
accounts is too high (EQ/TU only) - Ratio of balance to limit on bank revolving or other rev accts too high (EX only)
- Too few bank/national revolving accounts (EQ/EX only)
- Too many bank/national revolving accounts (EQ/EX only)
- Amount owed on delinquent accounts (EQ only)
- Date of last inquiry too recent (TU only)
- Length of time consumer finance company loans have been established (TU only)
- No recent bank/national revolving balances (TU only)
- Number of bank/national revolving accounts with balances (EQ only)
- Payments due on accounts (EX only)
- Too few accounts with recent payment information (EQ only)
Starting FICO Score 2007: >> >> 533 -- God those were some Dark Days !! 
