Large size as well. Italy, for example, has manageable debt (at least in the medium term), if the rates on it stay relatively stable. However there were default fears recently because of interest rates spiking, which could produce a negative feedback loop. U.S. rates are extremely stable, in part because the federal reserve manages them, in part because of general investor confidence, and in part because the market is so large that it's much harder for any sort of market moves to cause spikes (and nearly impossible to deliberately pull off a speculative attack).
The US also has considerably more breathing room to pay it off via taxation if they chose to. A country like Italy, where the government already takes 43% of GDP in taxes, has much less scope to plug any budget gaps by raising taxes than the US, which only takes 27% of GDP in taxes (across all levels).
The fiscal "breathing room" is there, but at European level (Ireland, Netherlands etc). Some US states also have an unmanageable level of debt, but it's balanced out across the whole union through the FED; in Europe this is not allowed by current treaties.
It's good to keep in mind that the debt level for US states is typically more in the 10-20% range. Since Euro member nations are - as far as money is concerned - basically in the same situation as US states, it's no wonder they're in trouble.
Fed (Federal Reserve) and Mac (Macintosh) are not acronyms, just abbreviations. Perl is an acronym though. That's not to say that it should be PERL - over the years, 'Perl' has become a word of itself, but the reason it's not is different from the other examples you quote, so as a series they don't make sense.
(Practical Extraction and Reporting Language, btw)
The US also has considerably more breathing room to pay it off via taxation if they chose to. A country like Italy, where the government already takes 43% of GDP in taxes, has much less scope to plug any budget gaps by raising taxes than the US, which only takes 27% of GDP in taxes (across all levels).