Edited Jan. 17, 2019 at 5:29 p.m.
Here's my take, and I'll try to keep it short.
The easiest way to classify defenseman is with a number. It's not the most effective, but it's the easiest. So...
Guy plays all situations, 25+ min per night, scores 40+ pts, 10+ goals most seasons, is on the ice for the biggest minutes of the game? He's a no.1
Guy plays well with with a number 1, might not drive play, might be less balanced than the no.1 but can handle the same workload? He's a no.2
Guy is really good at defense or really good at offense, but can't handle 25+ minutes per night so he finds a home on the 2nd pair? He's a no. 3
Guy makes zero mistakes if he's on a bottom pair, can play some special teams but probably doesn't get much of a chance, and wouldn't really explode if he did? He's a no.4
These aren't perfect, but it's much better than going to daily faceoff or leftwinglock or wherever, reading what pairing he is on today, and considering that to be his value in a trade.