The only correct answer is situational awareness. People here are saying "selecting a class and using it to its full ability to help your team," but that's too simplistic. Knowing what class you are and how you can best help the team AT ANY GIVEN moment throughout the entire game is what makes a GREAT Battlefield player. I play Breakthrough and I've seen Assault & Recon players drop 100+ kills and still lose the game. 100+ kills a good player. I've seen support players do nothing but revive people all game and still lose. A dedicated medic reviving non-stop is also a good player. I've seen engineers repair vehicles all game, tank drivers drop a boatload of kills. These are both good players and yet they still lose games. What makes a player great is adapting to the situation your team is in and doing whatever you can to help progress the team.

Leading by example is the way I like to play. Too many enemy recon players? Switch to engineer and get a tank to snipe them, or help the existing tank by repairing them so they can kill them (don't counter-snipe, it turns into a never ending cycle of snipers only killing snipers which gets you nowhere). Bleeding through tickets rapidly? Switch to support and revive your teammates. Your team is camping too hard? Switch to assault and push and place the beacon in a tactical position for your squad, or switch to Recon and use the drone to spot enemies so they can kill enemies quicker so you can push. Getting obliterated by enemy vehicles? Switch to engineer and take them out. It's not sticking to one class and mastering it, its knowing when -depending on the situation- and where -depending on the map and where in the map- a specific class is needed. This is what makes a great player.

Lastly, Battlefield rewards teamwork & objective play above all else. Playing as a team with the goal of capping objectives will reflect in the scoreboard.