The two factors that will hurt you the most is the age of the credit account, and your available credit to debt ratio.
Removing an older account takes that account out of the equation of calculating your overall credit score, which can hurt significantly, especially if that is the only, or one of just a couple, of open credit lines you have available.
Reducing your available credit will make your current debt look bigger than what it was before you closed your account. Going over a certain percentage for your debt to available credit can make you look less favorable to lenders.
[As stated above, closing a credit card does remove it from the credit utilization calculation which can raise your debt/credit ratio. It does not, however; affect the average age of credit cards. Even closed accounts stay on your credit report for ten years and are credited toward average age of cards. When the closed credit card falls off your report, only then, will the average age of credit cards be recalculated.]
And may I suggest getting your free credit report from https://www.annualcreditreport.com . It's the only place considered 'official' to receive your free annual credit report as told by the FTC. Going to other 3rd party sites to pull your credit report can risk your information being traded or sold.
EDIT: To answer your second point, there are numerous factors that banks and creditors will consider depending on the type of card you're applying for. The heavier the personal rewards (cash back, flyer miles, discounts, etc.) the bigger the stipulation. Some factors to consider are your income to debt ratio, income to available credit ratio, number of revolving lines of credit, debt to available credit ratio, available credit to debt ratio, and whether or not you have sufficient equity and/or assets to cover both your debt and available credit.
They want to make sure that if you go crazy and max out all of your lines of credit, that you are capable of paying it all back in a sufficient amount of time. In other words, your volatility as a debt-consumer.