1. To steal from Adam Smith, there is an awful lot of ruin in a nation. A country can absorb a tremendous amount of debt and spending without a collapse, and even in some cases with continued growth. Greece hit 100% debt to gdp levels in 1994, but didn't have (perhaps thanks to some fraud) major issues until the world started shaking in 2008, and I discussed Japan as an example in my most recent post. This is not to say that these countries were trouble free, only that the high debt loads looked more like a hindrance than a potentially fatal flaw.
2. High debt levels can exist for long periods of time, and so are "slow moving" crisis in that sense, but the end is very fast. The indicators that a country is approaching a crisis appear to likewise be slow moving for long stretches.
One rule of thumb that looks solid is a lack of improvement in debt to gdp levels during good times. Greece had decent growth from 1994 through 2008, but debt to GDP levels stuck at between 98% and 110% of GDP during that time. Think of it this way, if you left college with $50,000 in unsecured debt and started out making $50,000 a year and 10 years later you were making $100,000 a year but your unsecured (ie no mortgages) debt had also increased to $100,000 that would be a pretty bad sign for your finances, right? What are the odds you are going to on net be paying down debt in the future if you can't even maintain a level of debt while doubling your income? What are the odds you would be able to handle a major life crisis if your baseline is increasing debt? Japan also had some significant warning signs during the 80s. Their debt to GDP levels rose from 50% in 1980 to 70% in 1992 despite GDP tripling in that time.
This is very worrying for the near future as the US has been following this pattern. Debt to GDP was fairly stable at around 60% for the 90s and 2000s until 2008, during a period of relatively strong growth and then saw a large burst during and immediately following the crisis. While the expansion has slowed it has not reversed with a return to full employment, debt to GDP was higher in 2017 than any year prior to 2016.
This is a pattern not isolated to the US, I looked at 19 countries (why not 20? because that would have been more) including most of the top 20 world economies in terms of GDP and 16 of the 19 had increased their debt to GDP levels from 2008 to 2017 and 14 of them from 2012 to 2017. 10 countries had increased debt by 20 percent of GDP or more and 5 by 30 percent or more since 2008.
This is also not a series of small economies along with the US, 6 countries from that sample now have debt to GDP ratios of 85% or more and they are the UK, Canada, France, the US, Italy and Japan.