Bleach is one of those 'simple fun' shows. It's not super deep, but there's enough going on to provide some mild stimulation while enjoying the action scenes.

As a 'Shounen Jump!' manga, it has to have the prerequisite themes: Friendship, Effort, Victory. Friendship: "My friends believe in me so I have to win!" "I can do anything with my friends by my side!" etc etc Effort: "I lost last time, so I have to get stronger!" Victory: "Lol I went Super Saiyan/Bankai/Fox Demon on your bad guy butt."

It's all about the motivation of the young men (shounen). Value your friends, work hard, and you'll win at life. Maybe you'll get to swing a big sword around at some dead guys in the process.