Everyone views debt differently with varying degrees of adversity to risk and this will also change as you become older and your financial situation changes. To me debt (other than mortgage) is anything that you don't pay off in the month it was incurred.
If I think back to my first flat we were paying up 2 cars, mortgage, credit cards weren't being cleared each month, home improvements, store cards, white goods etc. The amount we were spending paying for finance was ridiculous and we were naively and blissfully unaware if either of us had lost our jobs or couldn't bring in a wage we would have been in crisis.
I am in my late 40's now (just!) and wouldn't, for example, take out finance for a kitchen. I avoid debt now, when we buy a new (to us) car we start savings for the next one instead of buying first and paying up, we have savings now for the bathroom to be done. But we can with a bit of planning and effort afford to do it this way, which means we have the luxury of not having to pay expensive debt costs (agree with pp who said 0% finance is an incentive marketing tool and the costs are hidden in your price)
Problem debt isn't when you get to the point where you cant pay, its when you don't have a contingency plan/insurance for when something happens and you can no longer keep up with repayments.