The gameplay is a major improvement over Serious Sam 3. For those not in the know, Serious Sam always had great enemy synergy, which I think is what takes an FPS game from good to great. I feel like this game has the best bosses since Serious Sam 2.
Music is great as always. From my understanding, many people hate the characters they added to it. I personally think they're a fine addition and make the game silly in a pleasant way. Their dialogue is great, mostly. The "side quests" are also an interesting addition. They kind of ruin the pacing but some of them are really fun.