For the Montreal Canadiens, losing Brandon Prust is a bit of a big deal. Prust's versatility to play both wings, and play anywhere in the lineup, has been a huge asset the last three years. However you could tell that things were changing last year, with Dale Weise getting more of the offensive minutes he used to get, and a couple of tantrums in the playoffs that saw Prust look bad publicly. Add in that Prust's offensive production has dried up, and he missed locker clean out day once the season was over, and it was just a matter of time waiting for the other shoe to drop.
Prust is still legitimately excellent on the defensive side of the puck, and the Canadiens will feel the loss of that next season, most acutely on the penalty kill. However Carey Price is so good, that defense just isn't as much of a priority for the Canadiens as offense, and Zack Kassian provides more offense than Prust does.
In trading Prust for Kassian, the Canadiens are essentially making a direct trade of defensive ability for offensive ability, but that's not all they're doing. Prust is 31-years-old, and he makes $750K more against the cap than Kassian does. Kassian meanwhile, is just 24-years-old, and will be a restricted free agent next summer when his deal expires.
Both players had discipline issues, though Kassian has faced the NHL's disciplinary committee more often than Prust has.
It's unlikely that Kassian has top-six scoring potential, but he could be a good start to changing the third line into a depth scoring line instead of what it currently is, which is more like "Please save us Lars Eller". With that said, Marc Bergevin and Michel Therrien always seem to give new players an audition higher in the lineup, so Kassian is unlikely to start out on the third line unless the Habs make another move.