They traded Thornton, Kessel, and Seguin in their early/mid-20s, for what largely sound like reasons related to not fitting into team culture. All highly drafted forwards that had produced at the NHL level and had many years of top line production ahead of them.
They're a team that has consistently demanded high effort, 200ft play, physical play, etc from their players for the past two decades. They also don't just demand it, but put their $$ where their mouth is and move on from guys if they aren't buying in.
Their ability to find Krejci, Marchand, Lucic, Carlo, McAvoy, Pastrnak, etc with non-top 10 picks is I'm sure largely due to the scouting staff, but I also think the team culture helps them squeeze more out of guys than other teams would. If you're on a team like that, you're more likely to eat/train right, not party to an absurd extent, buy into the system the coaches are preaching, etc.