I think what makes or breaks a Shounen series is how fleshed out the characters are. Plot is really very important as well. These two factors can really make a great shounen or a really bad one. This is partly why Rurouni Kenshin will always be one of the best shounen series around. Kenshin offers a likable cast of characters who all have elaborate back stories and also develop over time. This is also something Naruto does very well. What sets these two series apart though is the fact that Kenshin has a very compelling plot when Naruto's plot is far more weak. This is why people are drawn to Naruto. Kishimoto, while he is very skilled at creating likable characters, he really needs someone to be writing for him, because the series plot is a cluttered mess. It really doesn't get a set goal until 40 volumes in and it still isn't all that clear. Just rescue Sasuke. On the flip side, Psyren is an example of a series with a completely excellent plot but the characters are all very cookie cutter and have not really been given much backstory, beyond a little for Kabuto. Psyren's characters are really just devices in which the story can develop through. Well, that's my take on it anyway. Then there are just crap series like the ones that get canceled and fail to do both. Kinda like Kosen!