I'm a healer and I'm tanking a DK to find out why they're so bad, and I can't lose agro and cause wipes. What am I doing wrong?

And I'm tanking for DPS 2-3 levels higher than me with more damage.

Anyone got a quick list of typical class mistakes/misbuilds they use to teach failtanks when instancing? I only know bear and starting to get an idea on DK now.