The relative strength or weakness of a draft class is defined by the lottery picks. A draft's output distribution is typically exponential, with a ton of value in the first few picks before the line flattens out very quickly. Sure there will be big-time players after the first couple of picks, but for the most part, the stars go early. The average error tends to be greater at the very top than further along towards the middle rounds. In most draft classes from this past decade, this trend will hold up.
The outlier is 2012. One could easily argue that neither No. 1 pick Nail Yakupov nor No. 2 Ryan Murray are among the top five players from their draft class, and one could make a plausible (albeit somewhat difficult) argument that third overall pick Alex Galchenyuk isn't there -- or at least is not a clear choice. Fourth overall pick Griffin Reinhart is probably not in the top 20 at the moment. Note that these were not particularly controversial selections within the scouting community at the time.
It is debatable whether any of the players taken in the top four will end up ever being a top 5-10 player in the NHL. So what happened? Why does the top of this draft class look so disastrous at the moment?