What makes a pair of shoulder pads more protective? My best guess is:
1. Coverage or fit. A good fit, where the pads are properly aligned with where the impact points and prone areas are will protect better. This is a question of size and model. It has to be a good match with the wearer, or even the most protective shoulder pad won't protect as it should.
2. Material composition. There's usually a base of softer foam for comfort and some protection, paired with plastic and or denser foam for higher grade protection, resisting impacts of greater force. The top end pads are usually tricked out with what could be viewed as gimmicks: fancier foams, range of motion provided in dimensions that the body itself doesn't use, etc. You can get a sense of how protective the pieces are and where their respective strong and weak points are in person. It can be a little hard to get a sense for that shopping online.
3. Deformation depth: Just like how a golf ball collapses when struck, the stiffer parts in protective equipment (hd foam or plastic) also compress or collapse on impact. If there's more of a crumple zone in the high impact areas, there's ample space for the pad to bend inwards on impact (sometimes it's called absorbing the impact) and not reach the body of the wearer underneath.
I used to have light and slim shoulders (Easton Pro Light Design) that were light for their day but had minimally deep shoulder caps. Ultimately I replaced them because I was playing with some chippy idiots most of the time and wanted better protection against hacks and cross checks. The 20 years between that pair and my Warrior AX1s did a ton for improving shoulder pad protection and performance. I got the AX1s over Bauer Vapor APX2 shoulder pads because I got the AX1s for a great price on closeout. They're not impressively light compared to the APX2, but they are light enough that I don't notice them when I play. The fact that they are extremely mobile also contributes to that. When I took them out on the ice the first time, I was blown away at how much more mobile they were while also being much more protective than my previous shoulders. Although they don't pass the "looking cool in minimal shoulders department," I got over that by looking at how cool Fedorov looked even in bulky shoulder pads (we're about the same height) and figuring I wouldn't have to worry about looking cool anyway. Instead, I'm more concerned with whether or not I'm safe on the ice--before getting these shoulders, I didn't feel protected enough given the group I played with, sad but true--and whether or not I'm having fun. It's much easier to have fun with these shoulder pads on than my previous ones, and I have no motivation to change these at all until they fall apart.
For reference, I also wear full face coverage when I play. It has saved me a few times when I've closed on puck carriers faster than they realized and they stuck me in the face as I approached. No scars or dental work necessary.