Of course, you need to fully understand the risk/return profile of any leveraged investment you make AND be careful not to be overleveraged at any point. It's a fine line to walk, but it can be a valuable part of your investment portfolio.
However, if you are taking on debt for an expense (a vacation, a new car when you have a working one already, a new toy), then you are gaining nothing in return for the interest. You could easily wait until you have the money saved up, since those expenses have no extra return for purchasing early.
The worst debt, of course, is when you are taking it on because you simply have more expenses than you do income. This is often the source of credit card debt. If you are growing in debt but not growing in investments, you need to take a hard look at your lifestyle.
One can easily pay for a vacation or years of eating at nice restaurants every day all in cash, and also go into debt for an education.
Also, one individual taking out an auto loan and a loan from Sallie Mae isn't 1 good loan and 1 bad loan. You could've foregone the high-interest loan on the big depreciating asset and used the savings to fund the education instead of the loan from Sallie Mae. In that scenario both loans were probably a bad idea, regardless of the return the education nets you.
I think another key metric beyond interest rate and rate of return, is degree of necessity. That's harder to measure though.