Are the income tax brackets too high, too low? Are there too many? Too few?
I think the tax brackets of U.S.A are high comparing to other developed country. The following figure shows us the ratio between the highest break point of personal income tax and per capita income. In U.S.A case, the ratio is much higher than other developed country. It means the highest break point of personal income tax is higher than other developed country. Thus I think the tax break points of U.S.A need to be lowered.
Country
The highest break point of personal income tax
Per capita income ratio U.S.A
373,650 dollar
49,601 dollar
7.5
France
92,665 dollar
42,793 dollar
2.1
Japan
180,541 dollar
46,973 dollar
3.8
South Korea
270,660 dollar
23,680 dollar
11.7
[source : hangyurae news paper]
I think the number of the income tax brackets are appropriate. In South Korea case there are 5 numbers of the income tax brackets and in japan case there are 6 numbers of the income tax brackets. I think if the government use 6 numbers of the income tax brackets, they can divide people reasonably.

Is there an appropriate degree of progressivity (recognizing we cannot totally determine the true, actual, effective rate of taxation by looking at nominal brackets)?
I think the degree of progressivity of U.S.A is lower than other developed country according to effective income tax rates. The following figures show us effective income tax rates on USD 100,000 and 300,000 of gross income.
[ Effective income tax rates on USD 100,000]

[ Effective income tax rates on USD 300,000]

(KPMG, 2011)
These charts can show that