Macroeconomics analysis

Hello everybody!
Let me ask dummy questions!

1. Why is this ratio so critical: http://en.wikipedia.org/wiki/Debt-to-GDP_ratio ? http://en.wikipedia.org/wiki/List_of_countries_by_external_debt
For example, Luxembourg's value is enormous... still.. there is no bankruptcy or something...

2. Can anybody suggest simplest way to compare countries' economies? Say, 3-4 coefficients and their common sense, please.