What are your thoughts on this trade?

I think if Beaulieu want to play hockey is a good trade but if he want to drug is not a good trade

4th or 5th time a 1st rounder has been traded away by Bergevin, with Galchenyuk's future currently in question as well.
Not good for someone who has stated you find success by building through the draft, not trades or free agency.

Building through the draft is something that only works when you let your prospects play and put them in a position to succeed. Something Bergevin, in combination with Therrien, never allowed to happen. Bergevin has been too focused on acquiring veteran "character" players, which Therrien always put to use ahead of young players that need the ice time in order to develop. That lack of development and trust in the young players diminishes their perceived value, so when you do decide to trade one, such as Beaulieu, you are taking a loss.

A 1st round pick that has been developed properly and put in a position to succeed, will generally have higher value than the pick you used to acquire them. THAT is how you build a team through the draft. Use your players, raise their abilities and value, keep the key players, and use the extra parts as trade bait to cover the holes and/or upgrade your roster.

MB's strategy for developing through the draft has been an act of sabotage, and he places too much emphasis on character and experience.
You want Galchenyuk to be a two-way responsible center? Play him like one. Match him against tough opponents. Let him gain a sense of responsibility and gain some experience.
Then give him some wingers that don't get caught deep in front of the net (Gallagher, not always his fault, since you're allowed to maul him and delay his backcheck) or until recently, float their way back to defense (Pacioretty). Let him use his skill and have the players with less skill be responsible for when things don't go as planned. He'll figure out when and where to use that skill eventually.

Subban should still be a Canadien, and the team captain.
Tinordi should at least be a solid 3rd pair shutdown guy.
Beaulieu should be a 2/3 puck moving D.
Pateryn should be a 6/7 shutdown D.
Barberio should be a 4/5, PP D.
Sergachev should be entering the team this season as a 6/7 offensive D and move up the depth chart accordingly.

Add Markov and Petry to that group, and if they had been developed properly instead of left to rot behind the likes of Murray and Emelin, or trapped in the AHL (lost to waivers in Barberio's case), perhaps they would have reached closer to their potential. That could have been a young and solid defense core for a number of years, but most of those players will never reach their potential because they lost too much development progress early in their careers due to mismanagement.

Thanks, I appreciate the in depth analysis you have give me on this subject.

Do you believe that Beaulieu has 2/3 D potential???

I've always seen him as a 3rd pairing defenseman that was lucky to be in the NHL, but that could be for the development reasons you have stated above.

He makes rookie mistakes at times and was also brought up in Therrien's puck possession adverse "system". He spent most of his career so far either on the bench, behind players he was out-performing, in the press box, or on the ice throwing the puck to the other team on purpose, because that's what his coach wanted him to do.

He's going to need a couple of years in a good system with good development coaches to undo the damage that has been done, but he still has a shot at being a 2/3 imo. Hopefully he will be given enough opportunity in Buffalo to develop alongside their younger players and not get trapped behind aging D-men. He'll have to earn some trust from a new coach. He wasn't given any time to work with Julien in order to earn his trust. His development has definitely been stunted, so it might be an uphill battle though.

I think a 3rd round pick was a great price to at least give this guy a shot + he just had a 28 point season which would've been 2nd best on the Sabres blueline.