POJ had a fairly underwhelming finish to his junior career. Just didn't dominate like you expect a guy with that pedigree to. In particular his offensive game didn't grow as significantly as his early years would have suggested, and he was part of a Drummondville team that was viewed as a memorial cup contender, but lost earlier than expected in the playoffs due to a number of players underachieving (including POJ). Therefore at the time of the trade, his stock was probably in a bit of a lull, and while he didn't have a bad year in the AHL, he certainly didn't set the league on fire so that we all had to stand up and take notice (or so it seems, as I didn't see any WBS games).
Some of his peers in the Q seemingly surpassed him when looking at them from a distance (Gawanke, Dobson, Spence, McIsaac, Beaudin). However I don't really think POJ projects as a primarily offensive defenceman, as he plays more of all around game and doesn't really have that upper echelon offensive talent. In my view he would be better than those other Q d-men listed, with the exception of Dobson obviously. I saw him quite a bit when he was in Charlottetown and we played them in the playoffs when he was in Drummodville, so I've seen a fair amount of him over the years. Seems like a good comparable for him would be a poor man's Bouwmeester.
However, the point Speaking Moistly makes about Dumoulin sneaking up on us all a bit is valid for POJ as well (and many other defensive D-men). Unless you are watching these guys intently often, it's really hard to get a feel for how good they actually are since they often don't jump off the page at you in any given game.