@dmg,
Yeah, bass is a funny instrument. It's easy to get the rudiments and learn to play a simple three-chord song. You know, keep the rhythm and play the root note of the chord. But it takes a lot of experience to know how to play the right notes and the right amount of notes. The bass has a rhythmic and harmonic role and is one of the instruments where you can't afford to miss a beat. Guitar players are often hopeless bass players because it's easy to play for example lead-guitar fills, which is just wrong. But even though bass may seem "boring" compared with playing a screaming lead guitar, playing the bass can be extremely satisfying because every single note has such a great impact on the overall feel and mood of the music. It's really the bass that pulls the song in a certain direction. So, bass players are vested with a lot of power, so to speak.
As for John, he played what he was supposed to play, and he did his job. Simple, but effective. And "correct". But from an interview I read from years ago, I know that the way he talked about his style and repertoire, that he is pretty limited. Imagine the band wants to explore some more advanced stuff, rhythmically and harmonically, then he wouldn't know what to do. Hence, in my opinion his lack of versatility is a big reason why Mark went with Glenn Worf (and the session players for the jazzier parts of his soundstracks and Brother In Arms).